Mysterious ceiling stain -- please help diagnose
This morning, I noticed bubbling paint (approximately six inch by six inch) patch on interior edge of room about six feet from closest exterior wall on our first floor living room plaster ceiling. Noticed a few more small blistering patches (none bigger than 3 inches by 3 inches, two in addition to big stain) and a few water drop stains another foot to three feet away towards center of room. No dripping water anywhere.
I checked attic (third floor ceiling) and second floor ceiling and both are dry/no stains. There are no water marks on walls, baseboards, or floor of room directly above stain or walls surrounding stain. Venting for air conditioning system (connecting second to first floor) does run one to two feet from biggest blistering area, but air conditioning has not been on, and central air unit in other side of attic. No water coming from any airconditioning vents.
We have hot water radiator system that runs along exterior walls of house -- closest radiator (built into wall) maybe four to six feet minimum from biggest blistering area. Big blistering area has no obvious water staining. A few scattered water drop stains towards center of ceiling.
Who should I call for help? what else should I check first myself?