I have a mild version of this - usually I lean on a person's trappings, context, or they might hit a sort of LRU cache. In a crowded situation though all bets are off. Makes dating difficult - I've walked past girlfriends multiple times. Even if you try to disclaim this Prosopagnosia thing, there is a sort of innate offense you give when you don't recognize someone. The eyes give it away. Also if I look you up and down I'm not (necessarily) checking you out, I'm looking for distinctive characteristics.