Has anyone had this problem? I noticed a lot of postings on 'fishy smells'. Could this be the same smell? Can it be caused by a sewer problem? Do I call a plumber????
Mouse urine (use a black light) or a less than gas tight trap/drain.
A drain can be water tight and NOT be gas tight.
You can try to fill the sink as much as you can and then pull the drain plug and watch carefully for the slightest moisture at ANY of the drain joints (you may even need a mechanic mirror to check the back side).