I've never seen that happen with either of mine. I think I've only even seen the SCBS system kick in 2 or 3 times since I've had it, each occasion when parking up near a hedge or wall. I know the spec says it only works up to 18 mph (with a range of about 6m) - when I asked originally whether that was was 18mph absolute or 18 mph relative to another moving object (e.g. you're doing 45 mph, car in front doing 30, does it work), Mazda couldn't tell me...
A colleague who has adaptive cruise control on his Golf had something similar happen on a dual carriageway once. It was on a slight RH bend; the system picked up a stationary lorry which was parked in a layby thinking it was directly ahead and slammed the brakes on.
Alan