When resolving :"Choose a Space Marine. Spawn 2 Genestealers behind him.", I always took them from the Genestealers deck. Where exactly in the rules does it mentions that they come from blip piles?
This bugged for a really long time and until yesterday, I used to spawn GS from the deck, not from the blip piles.
Then I read this topic, went through the rules and found this: p21 for spawning rules.
"whenever a genestealer is spawned, the current player takes the top card of a blip pile and places it faces up in the appropriate position and side of the formation"
Also, on p21, it says " note that some card abilities spawn genestealers. These abilities may spawn genestealers in positions that do not contain terrain cards".
Finally, p22 " Important: Spawning genestealers are always drawn from the blip pile on the side on which they are being spawned. If the blip pile on that side is empty, then the genestealer is not spawned".
All in all, this tells me that the action of the event cards "they're everywhere" and "Out of thin air" count as regular spawning action and are therefore handled like regular spawning, which to me means that spawning comes from blip piles.
Which is good, because with "they're everywhere", I use to spawn from the GS deck, which would result in a seriously difficult turn.