@colin It can, it just seems the sensors aren't on all the time. They do go 16' out with respect to range (per TM), so I don't understand why the BSM is non-existent/so disappointing.
It appears sensors change sensitivity at varying speeds, which makes them super inconsistent. This is beginning to bug me.
The problem is that ultrasonic park assist sensors get less and less effective as your speed increases, especially when it comes to being able to detect objects that are moving relative to you. Most other cars use doppler radar in the 24GHz range to implement blind spot monitoring, which directs a beam of radar on each of side of your rear bumper, good for hundreds of feet (and they can see a rapidly overtaking car from 10+ car distances away and warn you if you're going to cut off a fast approaching car in the adjacent lane).
The downside of the radar based technology is that it requires you to be moving at a minimum speed. In my 2014 A6 it was 18mph+. In the 2015-2016 Audis and most Mercedes vehicles that was reduced to around 12-15mph. But still, it means when you are at a standstill in your lane and want to move into the adjacent lane, you are unprotected in a radar-based BSM vehicle, but that's actually one scenario that Tesla handles well with its parking sensor based "BSM".
I really wish the car would use a hybrid of these approaches. They each have their pros and cons, so I wouldn't say that Mercedes/Audi/friends got BSM done "right"....