My mom had a similar problem a while back on her 850 wagon. It turned out that her messy habit of spilling coffe on everything she came into contact with had ended up in a spill on her keys. Of course it never occured to her to clean everything up, just dry it off and go. Soon after her fob wasn't working properly. I popped it open and pulled out the rubber seal and buttons and found a nice thick gooey layer of old coffee that was causing the buttons to stick. The result was that even after she released it, the button stayed down and the fob sucked up the battery. Cleaned it out for her and no more problem.