There is always a deadzone in old abandoned cities, some are blocked as its uninhabitable due to poison gas or radiations within a given radius. Usually from military facilities or “secret labs”. And things they made are now roaming…too bad for fluffies. XD
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Makes me wanna play some fluffy D&D! Lol 
I always used to wonder what monsters in dungeons and other ‘sealed off’ type areas used to eat, since most animals would become aware of the dangers in the areas even if they could get into there ( don’t believe me? Its becoming pretty common for suburban areas and sometimes even small cities to become filled of deer right when hunting season starts in certain areas, almost like the deer are becoming aware that being in the wild forest is not safe during those times anymore. ).
BUT fluffies are way too stupid to avoid obviously dangerous areas, and thus fulfill an important task of keeping the monster population well fed XD.
In these suppose labs, they are in hibernation or stasis, cut off power and system malfunction most of the main cause of their awakening and escape.
Lets not forget the out of control portal doors thanks to idiots crying “in the name of science!”
Fluffy are so dumb I mean, what were they hoping to find it in the dead zone it literally has the word dead in it
You mentioned gas in your description, curious as to how Fluffies would deal with left of gas that had been used as a weapon.
Did this one:
The endless array of teeth, a madness of death over and over within the gut of the shoggoth.
Have fun, Fluffy.