It's possible that even though the handle is all the way down, it's not really released all the way. It's possible for the parking brake hardware to get rusted over time, things get stuck, shoes need to be adjusted, etc... Anyways, OP go get your braking system inspected by someone who knows BMWs. Draw this to their attention.
I agree this is very unlikely to do with having the pads/rotors replaced or sensors replaced. The parking brake system is separate from the discs/pads. It's actually drums under the rear rotors.