We too had to figure out what that beep was every 45 seconds in our garage. I wish I had been visiting this site more frequently at the time it occurred so we wouldn't have gone through so much investigation.
We thought that it might be our security system, so hubby took apart one of the sensors to check the battery and that was OK, but managed to damage it in the process. Well, our alarm went off and I had to quickly disable it.
Then, we thought it was the garage door opener since the sound was coming from that general location. The Garage door opener manual (found after much searching) didn't mention beeping in the troubleshooting area, so a question was posed to their support people on the internet. They replied that their garage door openers don't beep. They suggested that we look for a smoke detector as the culprit.
Well, then, we realized that the smoke detector would be located directly under the garage door opener (TM is closed up).
At first, we were going to pull the TM out of the garage and take care of it, but we have now decided to wait for Spring.
This is something we won't forget in the future. We DID remember to take the batteries out of the clock this year, however! Live & Learn!
Carol & Stan