I Drive brake wear question

[GR4]

Morning all,
I have a 2006 E61 520d. Can someone please tell me how the car knows when brake maintenance is needed. Is it mileage based, or does it look at wear sensors etc? I have asked technicians at work to give the car a quick look over before I take it to Europe on holiday and they say rear pads are low. But the I Drive says they have 25,000 miles left? I had new front brakes at last service with BMW specialist. Is it possible the garage reset a service interval for the rears at that time by mistake?
Thanks.

[phelix]

I don't know how the Idrive predicts brake wear but I don't trust it. My car is showing 60K miles till front pad replacement and it's been saying that since they were replaced 30K miles ago. And it says 26K tills the rears and the wear indicator is just starting to get touched as the pad wears down to a few mm.

[GR4]

Thanks for that. I suppose then it must just be a fairly crude system that gets reset by the garage at service time