I'm going to try and post this without giving away spoilers, so bear with me....

In 'Gatekeeper', the PC's have to track down the location of a specific planet for the Final Showdown. However, due to Imperial work in scouring planetary databases, the planet in question isn't in any navicomps or on any star charts. The players have to go to an old Jedi archives to find the coordinates. Once they find the coord's, the archives are rendered uninhabitable.
The players fly to the Forbidden Planet...and find that someone has followed them. This leads me to a couple of questions.
First: How did the mooks manage to get to the planet? The players had to track down an obscure Jedi archives to find the place, and the archives were in NO shape to be searched after the group leaves.

Second: The book states that the mooks follow the players through hyperspace. How could that work? I can understand if someone plants a GPS-style tracker on the players ship, but the book is kind of vague on the whole thing. "The mooks surreptitiously follow the players". Can you track and follow a ship through hyperspace? It kind of stands to reason that once the ship jumps, the best anyone can do to follow it is make a SWAG (Scientific Wild-Ass Guess) on where the departing ship went to.

I think it was probably an writing oversight (it happens), as jumping to hyperspace is usually a "well, we lost 'em" moment. Having the bad guys plant a tracking beacon on the groups ship would be how I would do it.

Some details are left out in adventures so the GM can fill them according to PC actions or his style of GMing.

One of the recent novels has a minor bit of damage to a ship leave a particular ion trail that pursuers figure out they can use to track the vessel (until the vessel discovers it and makes repairs). It felt more Star Trek than Star Wars, but it's canon if that matters to you.

I don't have the adventure yet, so I have no idea if your mooks are imperials, but reading your question I was reminded of something I read in the WEG Cracken's Rebel Field Guide, pg 62

Imperial XX-23 S-Thread Tracker

The star-spanning technology of the Holo-Net allows the Empire to operate a tracking device that can trail a ship throughout the galaxy. The Imperial tracking device uses a scaled down version of a Holo-Net transmitter, which broadcasts a signal that can be picked up by the Net's hyperspace S-Threads.

Imperial computers continuously compute the signal's direction and distance, telling the Empire what section of the galaxy a ship is headed for or what system it has docked in. The relay of information is instantaneous. The computers track the ship's general direction while in hyperspace. Once the ship-drops into normal space, it can be tracked to within one parsec. The tracking device normally signals a specific outpost or Imperial officer.

Addendum/Personal Cracken, Airen/General...

The trouble with these trackers is that the Rebellion doesn't have control of the Holo-Net technology. There is no way for the Alliance to scan for S-thread signals and see if we are being tracked; to see if a ship is bugged, you've got to search the whole thing.

Fortunately, the devices are too expensive for the Empire to use them more than sparingly. Since most of the Net has been dismantled (except for strategic military transceivers), the odds are good that a bugged ship won't cross enough S-threads to allow an accurate tracking until the ship drops into normal space.

2 more paragraphs not pertinent to the discussion

I am almost positive I remember references in the films to "calculating" all possible hyperspace jumps, when referring to tracking a fleeing ship. I would assume that due to mass shadows and known gravitation anomalies it might be possible to narrow down and make educated guesses on possible next destinations (although this number would expand exponentially by making short jumps and plotting courses from each new locations - kind of a forensic counter-astrogation measure). If your mooks have the manpower, maybe they simple sent grab teams to each possible locations.

I'm with HappyDaze on the Ion trail sounding bogus, especially since in Star Wars ships are entering another dimension to achieve FTL. At least with the above tracker a technology is being used that already sends information between the dimensions.

I'm going to try and post this without giving away spoilers, so bear with me....

In 'Gatekeeper', the PC's have to track down the location of a specific planet for the Final Showdown. However, due to Imperial work in scouring planetary databases, the planet in question isn't in any navicomps or on any star charts. The players have to go to an old Jedi archives to find the coordinates. Once they find the coord's, the archives are rendered uninhabitable.

The players fly to the Forbidden Planet...and find that someone has followed them. This leads me to a couple of questions.

First: How did the mooks manage to get to the planet? The players had to track down an obscure Jedi archives to find the place, and the archives were in NO shape to be searched after the group leaves.

Second: The book states that the mooks follow the players through hyperspace. How could that work? I can understand if someone plants a GPS-style tracker on the players ship, but the book is kind of vague on the whole thing. "The mooks surreptitiously follow the players". Can you track and follow a ship through hyperspace? It kind of stands to reason that once the ship jumps, the best anyone can do to follow it is make a SWAG (Scientific Wild-Ass Guess) on where the departing ship went to.

The following of the ship can be explained by force powers... seek or forsee, specifically.

Normally, but in this case the adversaries are not Force users. Either bounty hunters or criminals.

Fundamental principle of physics: Information cannot be lost, while the means to decode the information may not be available at the time, the information is there.

That means a sufficient technology could figure anything out (if you want to use Star Trek techmaguffins).

In canon though, Imperials often plotted trajectories of ships that went to hyperspace. This is possible because ships in hyperspace don't turn - the corridor through hyperspace they create is a direct structure between point A and point B, that's the reason for plotting the coordinates. What you do is effectively create a trans-dimensional tunnel from your launch point to your destination. That means the orientation of the ship as it launches into hyperspace can be used to "re-plot" the possible courses they took based off of the angle of their entrance into the hyperlane. As you have no idea of the length of the hypertunnel though, you need to develop a likely list of destinations along said trajectory, so it isn't 100%. That is also why you see the practice of making multiple hyperjumps - by exiting the hyperlane at a location that is in deep space, replotting, and making another jump, you can adjust the orientation of your entry hyperlane and thus throw off anyone attempting to recreate your jump.

I forget which book it's in but there is a reference to the Imperials being able to triangulate where a ship has gone. When my players asked about it I told them that all ships leave a somewhat unique exhaust trail that the Imperials are able to follow(trail), then using the "trails" direction they can roughly figure out which way the ship was headed(direction), if the Imperials know the type of the ship being tracked then they can guesstimate how much fuel the ship has (distance). Also due to hyperspace only allowing single line of travel this makes it fairly easy to track a ship, though this method is a hard way to do it due to the massive amount of time needed to calculate it all. Where this gets even more complicated is if the ship heads to a hub of travel. The exhaust of other ships intermingle with the tracked ship and makes it almost impossible to track. If that doesn't work for you though just put a homing beacon on the ship like in star wars rebels (faster and easier for the Imperials to follow).

Hope this helps.

I am almost positive I remember references in the films to "calculating" all possible hyperspace jumps, when referring to tracking a fleeing ship. I would assume that due to mass shadows and known gravitation anomalies it might be possible to narrow down and make educated guesses on possible next destinations (although this number would expand exponentially by making short jumps and plotting courses from each new locations - kind of a forensic counter-astrogation measure). If your mooks have the manpower, maybe they simple sent grab teams to each possible locations.

I am almost certain, the concept factors around determining the general direction of their hyperspace jump and figure out which systems are close enough along that path to be a potential destination for that jump. This has a major failing for this particular case in that the planet that is actually the destination isn't on any charts or databases so the mooks wouldn't consider it because they wouldn't know of it as even being a possibility. So they would almost certainly need some way to track the ship specifically, such as the tracker mentioned already.