And even when it does happen they'll still generally be out of sight...
I'm not sure i understand, does that mean they are supposed to be able to appear within sight (even if very rarely) at the start of a main complex map?
If not, here's a report of one appearing in sight at -3/Research (I think this is the right place to post this?) :
Beta 14
On (manual) seed "GreaterGarrisonLooping", i found a Behemoth barely in view at the start of -3/Research.
Only the right side of the Behemoth was in my FOV, so i probably could've¹ moved away directly on turn 1, so maybe that kind of situation is allowed? (Although allies would've likely been spotted, if i had any.)
¹I cloaked instead.
Here's the relevant part of my history log :
79600 | -3/Proximity Caves | Entered Research
79600 | -3/Research | Spotted B-86 Titan
79600 | -3/Research | Evolved 2 slots: Propulsion x1, Utility x1I didn't think to take a screenshot then, so the attached screenshots are of the layout after the fact.