You can't control irresponsible pet owners.And absolutely if I allowed pets and witnessed my tenant not cleaning up after a pet or littering you bet I would charge them. 

@Aly W. It's

I have been accused numerous time and it is always a power trip they are playing.

I find it interesting that people chiming in from the West coast have more to say about the validity of the source (I don't have an opinion on that, but I'm distrustful of the NYT - I wonder if these same people would be content if they were the source) and veiled accusations of rasicm. 

Landlords do not want to get in the practice of picking up their tenants rent otherwise landlord can not ever accuse tenant of paying late or legally charge late fees.

The exception (criminal cases don't count) may or may not be interpreted to cover the accuser(the landlord) in your jurisdiction.

This way, you are not accusing anyone of anything, but lets them know that people are watching.

The last thing you want to do is accuse one of your employees, without a shred of evidence, as a couple of posters suggest.  

At wits end, I send him a 7 Day Letter letting him know I am booting him and he pays.  

I would plan on exploring this from either Sheetrock wall on opposite side of opening up the wall from the tiled surface in the bathroom.You probably will not be able to keep the deposit in this case, and as you mentioned it was a 2 "flat" I am assuming you're in the greater Chicago metro and your L & T laws are very strict there.I would do as another post suggested and either have it repaired by current owner prior to closing to your satisfaction - meaning you are physically on the job site to witness extent of damage or lack there of due to water, or, renegotiate and ask for a concession for the issue - and this could be a moderately expensive problem depending on the length of time it has been leaking and given the possible location of the shower/bath to supporting wall etc should lumber need to be replaced.