Mysteries of my crotch: coffee edition

So I slept through my bus ride home today.  This is not an unusual occurrence.  What was unusual, however, was the fact that when I woke up my crotch and the lower part of my shirt were covered in coffee.  I had a quarter-full travel coffee cup in my bag, but it showed no signs of leakage whatsoever.  The bag was dry, the cup was dry, nothing in the bag even has coffee stains on it.

I have to imagine I would notice if my seatmate or another bus rider spilled coffee on me.  I am forced to conclude it was either a case of spontaneous generation or some new kind of stigmata.