I agree, it's probably a short.
Do you have an aftermarket alarm system? Mine was doing the same thing shortly after I bought it and after a little bit of digging I discovered that I had an aftermarket alarm and remote start system. This was not disclosed to me by the Lexus dealer I bought it from. Whoever installed it did a crappy job, so I put the alarm system in valet mode so it would stop going off and draining my battery but retain the remote start functionality.
The alarm on my previous car (a Volvo) would trigger if the battery was low. I'm assuming the system was programmed with logic to monitor the battery voltage and sound if the voltage dropped below a certain threshold, presumably in case a thief tried to disconnect the battery before breaking in. I guess this could be a possibility too. Swing by Autozone or something and have them check it out for free.