Is there any reason why this setup should be allowing sewer gas smell back into the sink? It only happens occasionally. I've cleaned the drain pipe in order to rule out smell from built up crud. Other sinks in the house exhibit the issue from time to time, however anytime I remove a trap its full of water.

Backpressure buildup perhaps somewhere down the line, either due to a slight blockage or poor venting. But I'm working from a perspective of a ship engineer. Not sure this would apply to landlubbers.

It may just be funk in the trap itself you're smelling. I know you said you clean it, but my bathroom sinks get a douse of chemical cleaner every few months. I think it's the toothpaste & hand soap congealing in there that starts to stink a bit. A little Liquid Plumber sitting in there for a few hours followed by a hot water douche & we're good to go for a few more months.

Trap should be blocking any sewer gas. Is there any drain above on a second floor? If there is another drain tied into that one, it may be sucking water out of the terap enough for gas to escape.

Have you ever disassembled it right when you smell it?

Reason I ask is that if not properly vented a toilet flush could be sucking it dry. Then when you run water it fills back up. This would explain the intermittent smell. Even if it's "sucked dry" there will still be some water in it too, but maybe not enough to fill it.

Do you ever hear it gurgle when a toilet is flushed somewhere in the house?

If it's windy outside the water can get sucked out of the P trap. Run some water in the sink and see if it goes away.

Some of my sink drains smell if I do not put liquid drain cleaner in them every few weeks. I have 11 drains & I only use 1 large bottle of cleaner. It does not take much to keep them fresh.

Check water heater sacrificial anode.

Sulfery smell, could be the water heater.