Stealing snail mail
From The Big Bang Theory Wiki
Leonard stole some of Penny's mail so he would have an excuse to go over and talk to her when he returned it. Howard says, "Stealing snail mail! Very old-school ... I like it."
Apparently Leonard still uses this ploy in the spring of 2009. After Sheldon asks Penny if she and Leonard are"friends with benefits" and she is shocked, he says: "Who knows what goes on over here when he pretends your mail was mis-delivered?" (0221)
