Possible leak in evaporator. Might want to add dye and freon. Drive it and try to find leaks if any. If after 1 week or so the compressor will not kick on because not enough freon you will know you have a pretty bad leak which you will have to locate.

Also if the system is empty of freon you could pull a vacuum on the system and see if it holds. If it doesn't you'll know you have a leak but you'll still have to locate the source.