With no networked reservoirs of data? Nothing. Information gathering is utterly reliant on information networks, and in the days before internet, those were networks of people rather than machines, but without some kind of network you just can't learn anything new. Your lore checks presumably represent either things you've learned from such a network in the past (especially for things like occult lore or archaeotech where it doesn't change, so you can learn from a library or university of some sort and that knowledge will remain current basically forever) or your connections to an active and ongoing network that you check up on regularly, even if you aren't able to check up on them right this second (so right now you might be stranded on some godforsaken rock in the middle of nowhere, but you've been checking in with your spymaster regularly and you can roll to see if he updated you about which Rogue Traders have been active in this system back when you are in port, because that's part of your routine and you don't have to roleplay doing it every single time just like you don't have to roleplay brushing your teeth).
So your Seneschal knows a guy who lives on Bastille. That guy has a bunch of spy minions, many of whom are probaly unassuming people like urchins or cabbies or others who can easily eavesdrop on passengers and report anything interesting they hear in exchange for a shiny nickle. Your Seneschal goes to his spymaster on Bastille and asks for a report, and the spymaster tells him everything he's heard, and possibly some inferences or deductions he's made based on that information (i.e. "little orphan Jimmy said that he saw some fancy looking captain asking about your mark two days ago, and Joe the port worker says he saw someone fitting Captain Scarypants' description pull in the same day, so I'd bet he's the one asking around"). Your Seneschal might then be able to make further inferences or deductions based on information gathered from other spymasters who've got networks in other places. The place where you learn about Rogue Traders is a place that's high-traffic with Rogue Traders, like Port Wander, where rumors and legends about the captains will be common amongst any population who works with the infrastructure they make use of (port workers definitely, maybe also pubs and the like). Some information might also come from earlier days when you were an urchin or a port worker and were hearing the information for yourself. This obviously applies to a very limited range of topics, but it can be a great character touch when visiting someone's home world to reference their career before being as big and powerful as they are today.