"YOU R BEING WATCHED!!"; Jerked One in Office Bathroom

This is not a troll.

Being stuck in the office late yesterday with little to do and no one around, my friend thought (for the only time ever) he'd save time at home by juicing the pickle in the bathroom.

Incidentally, he also had to loose shit, during which time the cleaning people came in to clean. Thus the original task was postponed, so he wiped and, after killing time at his desk, went to another bathroom which had already been cleaned.

After a somewhat long but not inordinate time, he tamed the shrew and left the bathroom. One of the cleaning people was for some reason waiting around nearby in the hall. He thought nothing of it, wrapped up work and went home.

This morning, he finds a napkin under his keyboard scrawled with "YOU R BEING WATCHED!!"

He's somewhere between nervous, since his move is stupid to do in the office, and outraged because the time a man spends in a deserted office bathroom is his business alone.

What, if anything, should he do? Ask the cleaning people if they did it? Pretend it never happened? Get a jump on this PP PI and report the napkin to HR? He's going to insist he was just shitting, if asked.