Windows
Cold glass sweats. A pipe does not care about the weather.
Winter condensation on windows is common in tight suites. It is not the same as a wet sill after a shower two rooms away, or a stain that grows in July.
Match the wetness to the weather and the room.
Condensation follows cold glass, crowded rooms, and poor air movement. A plumbing leak follows a fixture, a shaft, or a pipe run.
Wipe, watch, and photograph.
If the sill dries and only returns on cold nights, you may have a humidity issue. If it returns after a shower in another room, call for a source look.
Do not blame the window and stop there if the wall is wet.
A failed window can leak. So can a pipe in the same wall. Opening the finish without a plan mixes those repairs.
Questions people ask
Can a humidifier cause this?
Indoor moisture can. That still does not excuse an ignored stain that tracks a stack.
Is this a ProShield visit?
If the question is plumbing versus condensation, a source investigation can help. Window replacement is a different trade.