The act of urinating in complete darkness while in a bathroom, attempting to locate the toilet or disregarding it completely.
The next time your in the mall, picking up your dad from jail, or at loved one's home enter the bathroom and shut off the lights. Unzip and let it go. That's a mystery piss! You did it!
I took a mystery piss in the RCC bathroom today. The janitor is gonna be so pissed!