In some of the episodes, Columbo does little things which makes me wonder if they were ad libbed. When I watch, I look for them because it makes the episodes more entertaining. Examples:
"Now You See Him" - When the Great Santani gets out of the handcuffs, Columbo winks at him implying "I got you."
"Forgotten Lady" - He throws a kiss to the other detective thanking him for taking the his gun test.
"Murder Under Glass" - I don't know if this is the correct episode but he talks about how he was a Chef during the war. Which is true because he was a chef in the Merchant Marines.
Does anybody have other examples?
In Murder Under Glass, he talked about how his dad always did the cooking while his mom was in the hospital having another baby. The closest he mentions being a chef during the war is in Swan Song when he asks Tommy Brown about what he did during the Korean War and Columbo says that he was in that one. Tommy says, "Let me guess, MP." and Columbo responds with, "KP, mostly."
While there are some obvious ones, much of the genius of the show and Falk is that you can't often tell when he does it or not. Bochco explains it so well here at the 7-minute mark: https://www.youtube.com/watch?v=O2wOnLS7lqc
I'll keep looking for them though...