In our rental agreements, we use the security deposit to cover any damages once the lease has ended.

This is probably common among buyer prospects, too.

The other option is a water supply line, but that would typically resulting a steady leak and the damage would occur faster.Really think about what is above that space that could cause the issue.

I'm not an attorney, I haven't been hired to litigate damages and go back and fourth trying to prove or disprove who knew what when. 

And make sure you drive by prospective properties at night. 

Don't use overly rozy projections based on self-management and renting it out forever.2) Higher risk - how do you know which tenant caused the damages or brought in the bed bugs?

If they damage your property here, you're SOL!

You should bill them both for the damages, take pictures and go to court.

And more photos taken, same photos, and extra photos of any damage.

It appears you are able to hold her responsible to cover damages and lost rent.