Explorator Fleet
The Explorator Fleets are fleets sent by the Adeptus Mechanicus to explore less-known parts of the galaxy. Think of them as Rogue Traders, add in ship loads of tech priests, ancient space-faring technologies (which the priests keep to themselves, the selfish little...) and a big old focus on finding hidden technological secrets rather then booty and spoils and you have yourselves an exploratory fleet.
Their primary mission is to find lost technology, especially a working STC system (though printouts are also welcome). Often, they find other things, like new worlds and civilizations, or worlds with which contact had been lost centuries before, or Forge Worlds founded by a previous expedition. Often they find danger, which is why they bring along a detachment of Skitarii, just in case. The drive to find working STCs fills the tech-priests with the sort of (worrying) intense fervor so they risk huge fleets to find them.
Explorator Fleets tend to be crewed by the most curious of tech-priests, which means that when they find ancient tombs filled with impossibly advanced technology, they tend to poke around a little too hard and a little too deep, which wakes up the Necrons and causes lots of death (this happens in at least two of the Ciaphas Cain stories). It's a seriously endemic problem, probably compounded by the fact that some tech-priests worship the Necrons as angels of the Omnissiah.
The reason why the Adeptus Mechanicus isn't too keen on sending out Explorator Fleets is because they are worried about all of those nasty corrupting influences out there in space ya know. The lure of forbidden technology, the sight of tech heresies, xenos technology....it can all turn a tech-priest on their little metal head and at times not in the way the higher ups would like. Their reverence for technology means anything unusual (even a long lost type of electric toaster) can make them giddy and excited and since they are far away from the steadying hands of the senior Techpriests magos on the forge worlds (where they can monitor them big brother style) the chances of heresy are high. Add the fact exploratory fleets include the most open minded of tech-priests and well....such things are self evident.
So why allow such high risks? Why not allow the Rogue Traders or Imperial exploration fleets to bring back technologies, letting them do the hard work? It is because of greed. The Adeptus Mechanius are the biggest hoarders in the universe and are worse magpies than the Blood Ravens. They keep all the best technology for themselves and only really share the mass produced stuff due to their ancient agreements. While the imperial navy has to manually load torpedoes by hand with hundreds of workers, the mechanius have auto-loaders on their ships. While the Imperial Guard is armed with little, their skitarii counterparts are pimping it up with the best equipment. This is made worse by each forge world being technically it's own domain and often keeping secrets from it's fellows simply because they can.
The worst thing is the tech-priests don't know how the tech really works when they do get their mitts on it (relying as they do on instruction manuels such as the 'Dummy's Guide to...." series). So if an Explorator Fleet did find some unusual tech the imperium at large would proberly not know about it for a thousand years because between drawn out religious ceremonies, their desire to keep secrets for the sake of trolling, and just having no idea what button to press, it would take that long for the tech-priests to make any damn good on it. Admittedly, this is better than them pushing the wrong button and opening a new Eye of Terror.
On the plus side Explorator Fleets offer good settings for Warhammer 40k role-playing games. Want to be a tech-priest and his retinue of slightly colourful nutters exploring a new corner of the dark universe? Well now you can. It is only a matter of time though before one of them stumbles on a dimensional rift and find a borg cube or something.....("All hail our brothers in bionics! Wait, you want to do what? Well I can't join your collective I am already part of a priesthood....wait, wait what noooooooooooooooo. Oh you are injecting more machines into me? All hail our brothers in bionics....").
