Texas Yankee wrote:

I have double bathroom sinks - separate p-traps going into the wall for each
sink - each time I turn on the water in sink #2, there's a distinct rotten
egg smell, very briefly, that goes away once the water has been running for
a second or two - looking down into the trap from above the sink, before I
turn the water on, I can see standing water, so there's no sewer gas coming
in that way, right? What else should I check? Thanks!

#2 sink doesn't get used very often? Single handle faucet? Try turning
it all the way to cold and see if you get the smell. Most likely a
buildup of hydrogen sulfide on the hot water side while it's sitting
unused. Not much you can do about it other than using the faucet more
frequently. The hydrogen sulfide is generated in your water heater in
tiny amounts, but when the water sits for a while it can separate and
build up.

Pete C.