vehicle sits for a couple of days, the battery is drained?

I am experiencing problems with my Taurus. When the vehicle sits for a couple of days, the battery is drained and I have to jump start the vehicle. I took it into a mechanic and he replaced the horn relay, but it didn't fix the problem. He advised that he was not able to pinpoint the problem. I was told by another mechanic that it could be the GEM Module, but I had that replaced several years ago. I was told that I'd be better off trying to replace the sensors that feed into the GEM Module, but am uncertain how many and how complicated this can be. Any suggestions?