Brake Pad Question

[beekbmwm3]

Ok, so I've run into stuff like this before but I'm curious to know your response.

First off, I have an 06 X5 4.8is. I bought it used in December of 2006 from Penn and drove it back to STL, MO. Overall it's been a great car. Recently, I had the battery die twice on me...the first time I jumped the car and it was fine...the second time it was completely dead and I had it towed to a local shop (Dobbs in Stl). They called to tell me they were going to replace the battery and it should be ready this afternoon. Well, I got the call and the guy said the brake pads are down to a really low measurement and he could replace them within the hour at a cost of $650. Well, I never trust when they pull this stuff on me so I ask you a question...wouldn't the brake pad sensor go off before there were any problems? I know the technology isn't perfect but the guy told me I might not last another 2 weeks on these pads. In addition, I had the X serviced (Oil change) back 2 or 3 months, wouldn't the dealer have noticed this (if it is indeed a problem), and pointed it out to me? I'm just curious to know your response because I always question when they find stuff they weren't looking for in the first place.

Thanks!