Battery Draining When Car is Off

[RMARIANI]

My '89 GT was in storage for about 3 years. I've recently put in a low mileage, stock 5.0 just to drive it around. I've also done the rear, brakes and suspension. Engine and tranny will be next and finally paint probably next spring.

After I put in the new/used engine, I noticed my battery would drain with the car sitting for a week or so. I figured it was the old alternator so I replaced it and installed a new Optima red top. I drove the car about two weeks ago and it was fine. I went to start it yesterday and the battery was DEAD. Now I'm hoping its something simple like my vanity mirror lights staying on. I charged the battery last night and I'll check the lights later today. But if not (and I realize this is an impossible problem to diagnose on a message board) do any of you have any ideas of what might be causing the drain. I also think that my old alarm system might be a problem. I want to take it to an alarm shop and have it removed.