For a coat pocket camera, I probably would opt for one of the small rangefinders from the 1970s. There are some nice cameras from Vivitar, Konica, Canon, Olympus, Minolta and Rollei/Voigtlander. There's also the Zeiss Ikon S 312/Voigtlander VF 101, which are a bit uncommon but are solid rangefinders with a 40mm Tessar/Skopar lens.

I think the Pentax MX with the 40mm pancake lens is the smallest full featured SLR, but it has some weight to it, as a quality camera should have.