What have you included in your expanse that looked like plot hooks or lead ins to a bigger adventure that ultimately were just there to distract the players from what was really happening or make the expanse a more interesting place?

How did the players react to your red herrings?

I'm considering including a habitable planet with only three land masses. Two perfectly circular islands side by side and a large curved island below them.

I had Footfall be assaulted by a group of Death Cultists who were indiscriminately killing everyone to punish them for thinking they were beyond the Emperor's Light. The complication were these relatively uncomplicated cultists had somehow shut down/taken control of Footfall's defenses, and threatened to destroy the entire complex if their judgement was interrupted.

The initial response by my Explorers was to think that this was a targeted assassination attempt against them (nope), and then that it was a spearheaded effort by Hadarak Fel to seize control of Footfall by assassinating the Liege and then heroically coming to the rescue (wrong again), when in reality the entire assault was a distraction by an Irradial Cogitator that took the siphoned power from Footfall to attempt to remotely seize control of the nearby void ships for its own insane purposes.

Sometimes players just get swept up in ideas that have nothing to do directly with them, but they will be happy to assume it is about them and they will explore from this point of view. That's been a good rule of thumb for thinking up possible red herrings.

I'm considering including a habitable planet with only three land masses. Two perfectly circular islands side by side and a large curved island below them.

One of the NPC's, an old Explorator, died of old age. He sat on the riverbank and felt asleep. He never woke up. Of course my players are trying to tie his death to almost every little bad thing that happens to them...

A huge blast-door made of pure adamant on Naduesh, in the depths of one of the old hive cities. They didn't have the Equipment to blast through (used up 3 Melta-Canisters for their Melta-Gun). Behind it is probably nothing, or just a storage with rotten food.

I can confirm what Erathia and Commediant have said, the craziest red herrings that players go chasing after always spring from their own minds rather than that of the GM or writer. Anyway in the last adventure I ran some of their older enemies sent assassins to murder them at dinner, those assassins failed. Despite being on a fairly urgent mission they immediately started speculating as to how it fit in with the current adventure and promptly dropped almost everything so that they could go chase down the identity of the one who sent the assassins. Except here's the really goofy thing, they ignored the facts they uncovered revealing the assassins had posessed multi-tools and had been working as technomats on the ship for over a month. So then when the geller field malfunctioned because it's control cogitator had been sabotaged they were caught offguard so focused were they on determining where the knives were bought.

Some red herrings I've thought of using or have used:

NPC's killing/robing/beating up/bribing/charming/getting intimate with/running away from other NPC's, for no other reason than s*** happens.

Big Red Button on a Ork Cruizer's bridge which crashed landed on a Ice Moon.

Locked but empty security/stasis containers (the stasis containers themselves were rewards, but the PC's were like "blah" after wasting much effort unlocking them and discovering they were empty).

NPC ships raiding/destroying/colonizing/resupplying/passing through an area/world/space station (had PC's chance down an NPC ship just because it passed through an area they were starting a colony at).

NPC's asking favors/missions/dates to the PC's only to be killed later by an "accident."

Note: When I say random, it's only because I've randomly made "it" prior to the game, gave "it" a brief description off the top of my head, and jotted it down to introduce later on a note card/book/sticky note/etc. Sometimes I'm sneaky about it, and just hand the note card/sticky note to the PC who noticed it happening. What they do with the information matters not after that (well, to me anyways, the other players are usually like "psst.. what did he tell you?").

If you ever really just want to mess with your players just start having them make opposed willpower rolls or scrutiny checks in a totally normal situation land say..."Everything appears to be fine..."

That's what Paranoia's for, really. "You think you smell gas. It's probably nothing though."

Not just for Paranoia. A vital part of a GM's job is to observe the players and see where they are able to glean meta-information and where it is influencing their behaviour. If I run a pre-gen mission and have to look up sth in the book, my players automatically know that they are on track, that the NPC they are dealing with is one from the official publication, etc.

GMs need to put up some work to break their player's ability to predict.

Alex

I'm only running the intro section of my campaign at the mo but what ive done is write down key bits of info that certain characters already know or could find out and am using wiki entries and PDFs on my phone for other info as required. Between this and making stuff up on the fly from my vast 40k knowledge i keep my players on their toes as to what they should pay attention to.
I also like to fill my game worlds with lots of details that, if investigated, might or might not turn out to be useful. At the moment my players have 2 or 3 leads as to a prospective Endeavour they could undertake, but i'm keeping it fluid as to which NPCs will discuss which Endeavours so i can reward the group for good RP.

That's too complicated! There should just be a bulletin board near the airlocks on each station that is covered in Opportunities people have posted.

I jest of course...

That's too complicated! There should just be a bulletin board near the airlocks on each station that is covered in Opportunities people have posted.

I jest of course...

My Rogue Trader looks for those enough that I actually included one in some backwoods wayfarer station once.

That's too complicated! There should just be a bulletin board near the airlocks on each station that is covered in Opportunities people have posted.

I jest of course...

My Rogue Trader looks for those enough that I actually included one in some backwoods wayfarer station once.

That is AWESOME!!! Did it have a giant golden exclamation point?