I think it must be "Lady in Waiting." They aren't behind a curtain, but they are outside the house (Columbo tells her that to show her how pointless it would be to shoot him).
Thanks for your thoughts. I know that episode, I'm sure this is a different episode. I'm sure the police reveal themselves from behind huge curtains (or drapes as they're called in the US), in a large high ceiling room.
Although this was 30 years ago, so there's a possibility my memory is hazy and there's even a possibility that it may have been from a similar crime drama from the same era!
If none of the experts here recollect this scene, I guess that is to be true. :D
Maybe “A Case of Immunity”? The King is hiding behind a curtain (or wall) and comes out after the murderer admits his guilt to Columbo. No gun is pulled, though.
Lady in Waiting. Beth Chadwick pulls a gun on Columbo There were officers waiting outside; not behind the curtains.
Butterfly In Shades Of Grey has a similar ending, but outdoors. Some cyclists are blocking the road and when Columbo and Fielding Chase pull over to help, Columbo explains to Fielding Chase how he knows he committed the murder. Chase is about to shoot Columbo when the cyclists return and are revealed to be undercover police officers. They immediately arrest Chase for murder and one count of having the most hideous mustache since WWII.