I periodically experience a strong odor in the basement and in the first floor closet under the staircase. I have a half bath directly in front of that closet. The odor is prevalent in the closet and in the area of the basement under the closet. I do not have a floor drain in the basement and I have sniffed around all of the connections to the main drain to the sewer line. The odor is only in that one specific area of the basement and the first floor closet under the staircase

Several things come to mind. A crack in the stack or a faulty wax seal. It's possible for a wax seal to contain water and yet allow sewer gas to escape. You''ll just have to use your nose to find out. Good luck, tom

Do a smoke test. It is simple and will reveal the area of problem.

Good advice I agree with Doug, Thanks Doug, Tom