1. The plumber we use has been servicing the building for years, and seemed to have a good working knowledge of all the elements.
2. I think you're right about this.
3. We'll check the venting if the problem persists, but the fact that the snaking greatly improved it tells me that it is something in the actual sewer pipe, not the vent.
4. Installing another trap might be an idea -- and it wouldn't be hard, since the access valve I described is in a section of the pipe that is accessible through the building's utility room and isn't behind a wall.
5. This was one of my first thoughts, too. Not just that the pump was pumping the line dry, but since the problem appeared when the weather got cold I wondered if the sink wasn't used for several days, then the warm, dry air in the basement (with the heat on) evaporated some of the water in the P-trap and dropped the water level low enough to let sewer vapors escape up into the drain.
6. Yes, the bathroom has a vent fan.
5. This was one of my first thoughts, too. Not just that the pump was pumping the line dry, but since the problem appeared when the weather got cold I wondered if the sink wasn't used for several days, then the warm, dry air in the basement (with the heat on) evaporated some of the water in the P-trap and dropped the water level low enough to let sewer vapors escape up into the drain.
The trap losing volume from sitting is a distant possibility and usually takes weeks to occur (not days). I forgot to mention that to you in my last post, sorry for the omission. I'm not saying it isn't a possibility, just that it's a remote one. You may also want to verify the trap and following plumbing isn't angled downward more than it ought to be (the p trap should be almost entirely level if not totally level at the outlet side based on my experience). If the lift station isn't installed after both drains, then it could be pumping sewage into your bathroom sink from the kitchen sink - consider methods to fix that situation if needed.
The additional “U” trap, especially if the line is PVC and thus easy to open up & work on and the access is easy, would probably be a cheap thing just to throw at the problem and see if it works. “U” trap = same input and output height level.
I agree it’s a longish shot, but don’t be so quickly dismissive of venting issues. They can be oddball. Sometimes the particular height of a vent over the roofline and the particular nature of the prevailing breeze or an eddy air current over the edge of a building can generate an odd blow-down condition. And of course, you can’t see anything to diagnose. I’ve seen this fixed by adding a 1’ or 2’ length of pipe to a vent, which again is no big deal.