I have just had another alert, but it was another "fair cop".
IMHO there are two situations where legitimate warnings will trigger:
1. One tyre gets fast or slow puncture.
2. All tyres slowly deflate by unequal amounts.
The first type is what I reported in the OP of this thread, (slow).
The second type would not happen if you regularly check and reset, but I admit I don't. I can't remember when I last checked and just had a pressure warning and found the tyres were 25,25,25,30.
So after five years of ownership I am pretty confident in the system, and it has mostly done the job it is meant to do. The exceptions were false alerts in the first few months, and then three years later when I changed tyres, and had to have the OEM tolerances widened.