So, the previous dingbat owners (too many poor choices to list) of our home did renovations in the basement (no permit no inspections). They added a fixture in the basement bathroom adjacent to the laundry room (added a shower to just a sink and toilet).

Everything drains fine, flows: going clockwise positions are- the sump pump, then washer drain- in one room, then in the bathroom sink drain then shower drain then toilet. Directly above the sump is the kitchen and 1/2 bath- not above the problem bathroom

BUT there is a small amount of backflow into the basement bathroom sink, can leave an odor (bath is rarely used). I have tried to make sure the trap is filled with water, poured some borax for the odor. You can hear the sump kicking on. Above the sump on the floor above is another 1/2 bath with the toilet, as well as a kitchen.

They buried it all in finished walls, so cannot easily assess what the heck they did. It is not a large amount, not always, have noted it when the upstairs toilet flushes, but not always the cause nor always occurs. Seems as though it may occur when the sump kicks on- hard to catch problem in action.

2 things that confuse me: sump works, and shower (low point) is not the place where there is any backup and the drain is only 2 feet past the sink drain.

Any thoughts on this mystery that does not involve tearing out the walls to understand? Eventually will get to that I suppose...