Something is draining my battery while it's parked. How do I find cause?

What is your question?

Well, this just started recently. I have a car also and my wife can no longer drive and I am retired so the truck doesn't get used like it used to. It sits for weeks at a time. So, maybe this has been going on for awhile and was never a problem because driving it to work and back kept it charged. After sitting without use for a week when I first tried starting the truck there was nothing. The battery was dead! So I took it to a parts store to get it charged, but it was beyond charging. So I bought a new battery and of course, it started right up. No problems until it sat for a week or so. Now the new battery is dead. I did notice the lock icon come on on the dash when I jumped it. I hadn't seen that before. Right now I have the battery disconnected. Anyone out there have any advice?