SLOW tire leak..........?

My boyfriend said yesterday evening he had a slow tire leak. By the time i got outside this morning it was flat...to me a slow tire leak is a valve problem that takes place over a period of a few weeks. I cant feel anything coming out of the valve (tho like i said it is kinda flat) is it likely that there is actually some sort of pinhole puncture in the tire being as this has been very sudden? Or do you suppose a valve could malfunction out of nowhere. We've never had a problem with it losing pressure quickly before. (yeah, we fill it up every few months or so but thats normal to me...)