I live in a downstairs apartment in an apartment complex that was built over 40 years ago. A few weeks ago, I started noticing a very foul odor late at night in my living/dining area. I tracked it down last night to the wet bar area. There is a small sink which has not been used in the 3 years I've lived here. The faucet was removed and the sink covered with a board so it could be used as a shelf for my stereo. When I opened the cabinet below the sink the odor was very strong. There was no evidence of any leaks from the pipe. The odor smells like sulphur and sewage combined. There is no odor whatsoever until late at night - after 10PM. Therefore, since maintenance doesn't work after 5PM, there will be no odor present when they come inspect.

Does anyone have any idea as to what could be causing this odor to appear only late at night? Thank you!

What you are smelling is sewer gas from a open trap. This gives your house a direct connection to the city sewer. Sewer gas's harmful to your health and the methane content makes it explosive.
The reason you smell it at night is because sewer gas descends as it gets cooler.
SOLUTION:
Pour a quart of cooking oil or antifreeze down the bar sink drain.
Then check the trap to make sure it doesn't leak.
Good luck, Tom

Tom, thanks for the response. Now I'm really worried! There was no odor last night. Would sewer gas appear off and on like that? Do cooking oil and antifreeze seal it off or something? Why would it just be coming from this one sink?
Thanks again.

Would sewer gas appear off and on like that? Do cooking oil and antifreeze seal it off or something? Why would it just be coming from this one sink?.
Hey Joe,
Sewer gas raises and falls with the temperature. We advise cooking oil or antifreze because they will not evaporate.
You smell sewer gas from this sink because it hasn't been used and the water in the trap has evaporated leaving a direct open connection to the city sewer. Good luck, Tom