Take any screengrab with two people in it. One of them says a bit of small talk to the other, which is then followed by an awkward pause. After the pause, one of the talkers asks the other "Storeroom?" in a bit of sexual propositioning. The other talker almost always responds with "Sure.". They then head off to the storeroom together.
Not clear on who started it, but it began in the early days of Inventing Situations and had quite a run for a week or so. Although the usual format included small talk, awkward pausing, and an invitation for two to the storeroom, it also branched out to include generic jokes about storerooms being dens of sexual depravity.
(This, though nothing yet fully confirms it, is most likely a small reference to the Simpsons episode "Grade School Confidential", which ended with Principal Skinner and Mrs. Krabapple having a similar exchange.)