Not a big deal - thankfully, had the tools and found it before it fell off!
Piaggio's torx are not my fav fasteners - but I learned to keep a set of "L" torx tools with me at all times - under the seat.
I knew rain as expected on my ride - so headed to a park to take a look at what needed doing.
I discovered that the case-to-plate torx were tight - so it was an issue with the plate. Removed the case and found that the 3 plate-to-scooter screws were loose.
The issue was - these were not torx - but allen (hex head) screws!

Was surprised & happy to see that, years ago, after I installed this case - I had tossed a set of hex head tools under the seat - just in case!
In short order things were put right .....
The case had been on the scoot since @ 2019.
O.S.