[RWebb]

take a camera in next time & snap some photos of the egregious behavior - post it on youtube
(you can be stealthy if you use a cell phone with camera & pretend to be talking on it)

re the trash can - there should be one next to the door -- these doors usually auto-close, so if not he has no ability to walk back to a can; sounds like your setup may be different

since I have the microbial knowledge of a biologist, I'm pretty careful in public restrooms - if there is no trash can within my very very non-NBA target distance I WILL toss the paper towel on the floor

and BTW the chance of disease spread is a lot less in an office bathroom than in a public one - but yes it is still high, esp. for flu or rhinoviruses ('colds')