Q: Battery Keeps Dying

My battery keeps dying and I don't know why. I first replaced both the battery and alternator. Was fine for a couple of months and then started dying every day again. I turned in my battery for a new one because the fluid level was low. Was fine for a month, started dying again. I've done several parasitic drain tests, nothing comes up. So I had my mechanic friend check out my car, and he took out my alarm wire (which we thought would be draining my battery because my alarm was going off sometimes for no reason) and charged my battery for a little bit, then my car was fine for a month and a half. Now it's still dying and can't really hold a charge for more than a day. I need to fix this problem now before I need to buy another alternator and battery again.

My car has 167000 miles.
My car has an automatic transmission.