I do read what you've written!
Yes - quite right - "... it just senses movement". And to do that it must sense that a wheel(s) is revolving you'd therefore agree, as only such a sensor could tell it it's moving. (I suppose its GPS could but hardly likely.) In fact it has to travel a distance, if short, for those sensor(s) to register a suitable number of pulses to send to the CPU - so there will definitely be a minimum distance involved. And even more to the point, it's highly likely using the speedometer's sensor, wherever that gets its signal from, because it says in the manual "The Downhill Drive Support is available in the speed range of 2 km/h up to 30 km/h." So if knew which sensor the speedo used; how many pulses were supplied per wheel revolution, or prop shaft rev, or whatever that sensor is sensing; how many pulses the CPU was programmed to require before it calculated a speed; I could extrapolate from that, knowing the circumference of my M+S tyres, how far the car needed to to go for the hill descent to turn on or off. Which would equate to wheel revolutions. Yes?
Obviously the inclinometer (or possibly just a pair of tilt switches?) will need to register that it's over its pre-determined angle and the hill descent needs to be used. Incidentally the manual says "... the downhill gradient is at least 10%", so, to correct you again when you said above "no-one knows what angle is needed", there is a published angle even though my car seems nearer 7.5º which converts to a 12.5% slope.
All of which actually answers my original question which was: why, when I come to a halt, is the hill descent system still pressing the pads against the discs. The answer being that it hasn't had enough distance to re-calculate that it's no longer (movement+inclinometer) on a slope and can switch itself off. So I've answered my own question which I now wish I hadn't asked in the first place. And I now know that the easiest way to reset the hill descent system when I've stopped is to switch off, switch back on, count to three, and switch off again. Voilà.