Welcome to the forums James.
Are we talking about the Key FOB'* for your 2007 GMC Sierra Classic?
I cannot think of anything that could drain the batteries that quickly, not given what you have stated.
I did read that some people claim having a cell phone too close to the key FOB'* can cause battery drain, maybe somehow it is trying to communicate with the transponder, not sure how though.
Also read strong magnetic devices may be responsible, but I am not sure how this could happen.
Do you keep the FOB'* near a cell phone a lot, or near any battery chargers?
2006 Pontiac Grand Prix GT with SC
1996 Buick Park Ave NA