I was using the bathroom, and was in the stall when I heard sounds that were very suggestive of pornography coming from the other stall, which was closed. To be clear, there were two voices and they sounded like they were electronically generated, like from a cell phone. I was not being quiet, and the sounds persisted while I washed my hands even though I was fairly certain the other person was aware I was there. I have no idea who it was, I went back to my desk intending to tell my direct supervisor but he was nowhere to be found. I didn't want to flag down a random manager, and HR is on the other side of the building. 

The person is gone now, I am hesitant to bring it up now. Is there a compelling reason to, or a reason to drop it? (at least as long as it is not a repeat incident).