For a couple of years I had a bad smell from the sink that I remember from childhood - dirty floorcloths that were left wet outside the door for years, full of bacteria obviously. Treated the sinks with caustic soda loads of times, poured down gallons of bleach, even called a plumber who said his wife complained of the same smell but he didn’t know what caused it!! Eventually I called in another plumber who said it came from one of the washing machine pipes, he altered the angle I think of one of the pipes and, hey presto, it vanished overnight. If I’ve been away for a few days I smell it faintly on return, but it disappears with the first wash.