Oops... Its not literally "Bandit Warehouse" bounty hunt mission, my mistake. I forgot the name, but generally its a mission with one building that has some air rockets for your crafts and deadly, but immobile gun placements outside. It appears after researching "We need craft weapons" (or something like that) and then disappears after researching some other craft weaponry project.
It's too late for me but maybe someone other can reap the benefits of what I managed to extrapolate on how get a
Stormrat. I'm too dumb to confirm exactly via rules examinations so I'll go off what Dude said, pedia and some logic hopefully.
Mission that spawn them is called
!Highway House!.
Research that disables the mission is
Contacts: Krazy Hanna.
So Stormrats are then only available between those two research topics.
What makes them easily missable is both topics are unlocked by
We Need Craft Weapons! (with Hanna having bit more dependencies) so it possible to research Hanna not that long after Highway House, disabling it before it has a chance to spawn (not before following month at earliest). And unlocking Hanna to be able to buy craft weapons ASAP is impossible to resist. So that's why I think I missed them.
UPDATEHanna apparently no longer (as of L7) disables Highway House. Mission finally spawned in my game in 2nd half of 2604, way after I completed all available research. I wanna believe Dio intervened and changed said mission spawn rate, rather than me being so damn unlucky that I had to wait 4 years to get a stinky Stormrat :)