Sort of like "I see" is when he asks the killer about something that would clearly point to his guilt, and the killer suggests some other scenario about the clue, and Columbo says:

"Well that would explain it."

Meaning of course, well, in theory that "would" explain it -- but of course we both know the real explanation, which is that you did the murder.

Except that goes without saying, so usually Columbo just pretends he buys the story, and waves his arm and begins to leave. Then comes back again.

There's at least one expression that's popular among the killers themselves. It's telling someone they have "the subtlety of a train wreck."
Dexter Paris says it to Columbo himself, Emmett Clayton says it to Berozki, and there's at least one more time I can't think of.

The third "trainwreck" quote was in Double Exposure. Keppel says it to Columbo in the grocery store when he asks Keppel if he was about to be canned.

"My wife, she's a big fan of yours"

"but that's just me"

Gee that's funny

I was in the neighborhood

A head of lettuce and two quarts of milk

Murderer "what have you got to go on?"
Columbo "At this stage, not a lot"


Walking into someone's home "This is a great place you have"

"well I'll be a son of a gun"

"My wife will get a kick out of this"

Whilst searching every pocket..."what did I do with that, it's here somewhere"