Mysterious leak dilemma.

In the basement, occasionally (weeks apart), one or two drops fall off the
plastic drain elbow directly under the toilet upstairs.
Apparently there's no leak coming from inside the pipes. Nor is there water
seeping under the toilet (floor is dry). The leak doesn't correlate with
usage. Repeated flushings do not show any leaks. Ditto for other fixtures.

Could condensation be responsible?
Vent stack leak?
Any other ideas?


TIA!