"Ransom for a Dead Man" - the very first scene with Columbo in it features him searching in the dark on the steps of Leslie Williams' house for his pen/pencil.
My favorite scene of this nature is actually (from "A Friend in Deed") when he "loses" his cigar underneath the seat of Deputy Commissioner Halperin's limo. The line where he says, "Come here you little rascal", cracks me up every time. Evidently, it cracked up the other actors, as you can see the guy in the front seat laughing away and I don't think it was all acting. Anyway, it was a nice twist on Columbo always losing things (his pencil, his notebook, etc.)
this pattern gets set up early in the series right from the beginning from Prescription Murder when he says that his wife gives him a pencil every morning and he always loses it.
And both of them forgetting about the pen gives Columbo an excuse to get the ball rolling. The episode also ends with Columbo in need of a pen, which is supplied by the officer, although Columbo didn’t ask for it.