Not so much a bug but an unintended side-effect. With Wander Spawns on, it’s possible to clear out areas you want to explore simply by walking up to them (close enough to get the zombies to spawn), then walking away. Because spawned zombies now de-spawn and turn into wander spawns, these zombies then move to the overmap, where they’re free to wander away, literally disappearing out of sealed/enclosed buildings. You can make an entire hospital of zombies disappear, which can make looting outer buildings a cakewalk. As far as Z-Levels are concerned, this only seems to happen at Z-Level 0 (surface), regardless of whether Z-Levels are enabled or not.
EDIT: When using a name as a verb, make sure you can spell it correctly. facepalm
Zombies don’t natively spawn underground, and for the most nothing from the surface does.
It is possible for swampland critters or ants to, due to how their native habitates work. So seeing ants or giant frogs in a lab isnt completely unreasonable. They after all can live there and are set to spawn within a radius of their hill.
Zombies typically do not, however they do spawn in basements and other select underground locations which can provide a never-ending stream of them; though it seem pretty rare since the stable 0.C.
However, yes, wandering hordes can spawn over 100-thick right in front of you. Ive definitely opened doors before and found 100+ zoms just waiting patiently for me to bumble on by.
What’s more, they can and do spawn right in front of you. There’s no restriction on it. And its not just zombies; zoms are just the ones that spawn in large groups most often.
Ants can spawn in gutted out cars you are trying to salvage and attack an otherwise safe character.