Hmm, there's definitely Nadia Donner in "A deadly state of mind", the murder by hypnosis, and also Harry Alexander in "A stitch in crime".

Lisa Chambers' murder from "Double Shock" doesn't get resolved on screen. Also Charles Clay's murder from "Last Salute".

Both murders may have been solved later, off screen or maybe Columbo got confessions. It was either Dexter or Norman - or both - who tossed Lisa over the balcony of her apartment. And Charles Clay would have been killed by 'ol Tisn't himself as well.