Seeing this thread bumped lets me make two corrections to this observation of mine. First, Sierra's credit is at the back end of the credits and he does appear in the episode as a fellow detective Columbo calls by name "Lou" when he arrives at the crime scene. He's wearing an obvious toupee which camouflages his appearance a bit.
Has that pesky Arlene Martel credit ever been solved?
Someone on the Ultimate Columbo Site message board finally spotted Ms. Martel in Double Exposure. When Kepple goes to plant the gun in the Norris house, Mrs. Norris leaves just before he goes inside. Coincidence? Long wait? No...she gets into a car with Arlene Martel. Kepple's confederate, Tanya Baker was obviously helping clear the way for him. The rest of the scene must have been cut.