I moved into my apartment a couple of years ago, and got my first snake right after that. The lease provided for cats and dogs, with extra fees of course. My apartment manager met one of my boys when I was out letting him play in the summer grass about 8 months after I moved in. She was not pleased. I took the time to look up my local laws, so I can now quote the ordinances in my city: Chapter 15-1, Subsection 15-1.1 and 15-1.2e, which state that I am clearly allowed to own my animal within city limits because he is non-venomous. That quieted her down right away. Plus the maintenance guys and the apartment supervisor (one of the big boys from corporate) were just over here and they each wanted to hold my snakes.
I suggest knowing the ordinances in your city, and if the lease is vague, it's trouble. Almost all of my neighbors in my 24 unit building know about my boys, and they visit me when I bring them outside for grass time.
How to prevent trouble:
#1 install plastic sweeps under the front door and bathroom door. Keep your bathroom door closed.
#2 find any heating or water holes and stuff them with generous amounts of steel wool.
Done. Now they can't get out through the wall or drains.