interior baseboard moisture

I completed a inspection block home, interior closet had approx 5 ft of wet baseboard its adjacent to a hvac vent return (in closet) which is dirty no plumbing pipes anywhere near moisture any ideas on source and property is on full slab.

Was it next to the air handler?

no air handler was in attic 25 ft away and discharge went opposite way as well.

did it have drapes going to the floor stopping air movement .

did it have drapes going to the floor stopping air movement .

no isolated vent in closet no drapes inner wall with dirty vent filter in return
I am thinking with this home being vacant and with dirty filter might be creating condensation and or a leak behind return and water is dripping make sense?

Not on the return, they don’t sweat. What else was above the wall? Near a bathroom?

bathroom was about 15ft away thats whats difficult other than the vent no source of water no blockage in middle of home inside closet wall air handler in attic 25 ft away and discharge flows opposite way of home from handler
house on slab water heater 20 ft away and all piping in attic and none in area.
completely isolated moisture dead center of home and scanned wet and wet to touch.

Did you check the water meter for spinning?

After checking the water meter for movement, check attic above area for any roof penetrations, (plumbing vents, roof vents, etc.) Also check ceiling of closet for wet or damp conditions.

completely interior like as in the middle of the home? Cat/dog pee

What was the Plenum material?
P trap on condensate line?
Cap on condensate cleanout?