When you go to a more anonymous or out of the way bathroom rather than the one you usually frequent solely to take a dump (grumpy) with more impunity due to intensity, privacy or psychological reasons.
"Hey, I was down on the 15th floor and I caught Jeff coming out of the bathroom, he must have had some bad food for lunch or childhood issues for him to take a secret grumpy."
by Senor Grumpy August 29, 2011