said by Mr Matt:How does the condensate drain line run from the air handler? Does it run over the area where the wet spot is. When cold condensate runs through the condensate line the outside surface of the line gets cold and collects condensation. If you have enough condensation you can have drips and a wet spot.
I was thinking along the same lines as you are with the exception of maybe a leak in the condensate line.
Either way, the ceiling really needs to be opened up and checked. The cost to repair that size of a spot is really not that much money in the scheme of things.