Help needed with stain on ceiling near air vent

last month

I have these stains on the ceiling, right at the air vent in two separate rooms. These stains are brownish in color, not wet. They only appear around these vents. Tried cleaning, won't come off with out removing the paint . I've had the roof checked for leaks. Talked with the AC guy and he has no suggestions.

I have central air, gas heat. I have a humidifier installed, so I think that it may be the culprit. I had the humidity turned up too high when it was first installed, thinking that may be the issue, but my AC guy doesn't think so.

Any suggestions?

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