For me, it's akin to the cold and staying indoors in the winter in NY. 

A wild cat has the ability to eat, drink, and defend itself for its entire lifetime, but bring it indoors and feed it for a few weeks and it won't leave because well heck, life is a heck of a lot easier in here with this person doing everything for me.

Is it possible I need to find someone to seal the foundation of our building to ensure they can't get in from outside, and then once the indoor population is isolated try to stamp them out? 

Hi, yes, kind of, once you divide utilities as mentioned, you have some compliance issues to deal with, evemn compliance issues that won't really pertain to you as dividing a garden hose with a meter on it.

You can't just say "meh, I think I'll just garden today instead of work!".

We used to have "Smog Alert" days fairly frequently, when you were supposed to stay indoors as much as possible. 

The property is off of Garden of the Gods, west of I25 so it is in the burbs.  

Luckily, all of my tenants have had well-trained, indoor pets that had already lived through their puppy years in someone else's property. 

Weed is legal in-doors (and users are very normal young people), but it's still hilarious how embarassed tenants are when the smell is still hanging around while you inspect the unit.

I have be involved with the purchase of 4 different 25+ garden style apartment complexes and 3 never made it to the MLS and the last one was on the MLS for maybe a few hours before we had it under contract.