For 20 TV years, the detective in the rumpled raincoat has discovered whodunit on every case he has had. Strange, then, that Lieutenant Columbo can't seem to locate his wife. Mrs. Columbo, who appeared in a 1979 spin-off flop starring Kate Mulgrew, has never surfaced on Columbo itself. The closest substitute fans can expect is Shera Danese, the real Mrs. Peter Falk, who plays Trish Fairbanks on the April 29 ABC movie, Columbo and the Murder of a Rock Star. ''I've gone from being the murderer's secretary to the murderer's assistant to the murderer's law partner,'' says the former Miss Pennsylvania of her biggest Columbo role to date. Danese confesses that her husband is exactly like the humble, fumblesome gumshoe he plays. Does this mean he's a little, ah, disheveled? ''A little? A little?'' His wife of 13 years howls with laughter. ''One day he went out of here to work he had his fly down, he had cut his chin so he had a piece of toilet paper hanging from it. He hadn't combed his hair and he hadn't tucked his shirt in.'' And his raincoat collection? ''He's actually got a Thierry Mugler and a Giorgio Armani, so I've tried to change that part of the image a bit.'' Despite such home-front proddings, Danese insists that she likes working with her husband. ''We don't fight on the set,'' she says, ''I wait till I get home.''