I have had (since the advent of the use) several upper level riders in warmup at FHI dismount without unclipping. (Probably average one a year.) I keep a lot of little extra things with me at warmup (because it is so far from the barns, parking lot, and vet box) for the grooms, but was really surprised when someone asked me for a spare cannister once! (really?)
I have had less numbers -- perhaps only one over the last two years -- forget to clip the vest [that we noticed] on the way over to start box, but I am not always the one checking riders for that. I have a lot of other more important FEI stuff I have to watch for. I do write it down on my white board with the numbers/times -- a little blurb about unclipping if you dismount -- since I was expected to have an extra cannister.....baby, your SADDLE costs more than my car parked here that I am working out of today....not likely! Since it's not an FEI thing I don't pay attention to it. Now that there has been this discussion I will probably start watching that lanyards are not looped and swinging on the chest, but I honestly wouldn't say anything unless a rule required me to check or something -- I leave riders alone as much as I can in warmup, and try and check with their grooms or ground people if I have a question.