The Out of Thin Air event card contains the following text:

    Instinct: Choose a Space Marine. Spawn 2 Genestealers behind him.

Suppose a marine is facing left and the right blip pile is currently empty. Is it legal to choose this marine and thus avoid spawning any Genestealers (since the appropriate blip pile is empty)?

okorz001 said:

Page 14 of the rules uses the very card as an example...

And no, you have to choose a marine facing right if you can.

Dam said:

Well... now I feel dumb. I missed that little blurp about always choosing valid targets for Event cards. I assumed that since only some of them say "if able" that it wasn't always mandatory.

Thanks for the answer.

 I always played that things that didn't refer to the blip piles spawned from the Genestealer deck, which is wrong. It solved this issue, but sure did screw us over with tons of "free" genestealers!

Where is correct to draw? Blip pile or genestealers deck?

 Always from the blip piles.

Uncle Kulikov said:

Unless otherwise stated; a couple cards say to spawn from the gene-stealer deck. Most, however, do state to spawn from the blips.

Now I have a question: how is one of the gene-stealer decks emptied out? Wouldn't that mean you would travel, which would create new blip decks?

The only way I can see this happening is if you are at the final location, and yes, it would make sense to then spawn from what blip was left.