I understand your concern (I had it myself initially) but here's what I want you to know:
First of all, in all the years I have been hanging around here, we have had exactly *1* scooter that was "stolen" - and that turned out to be a case of "mistaken identity" where another person who had rented the exact same model of scooter had driven away with our friend's scooter quite by accident. Eventually the situation was sorted, and the "stolen" scooter was swapped for the correct one... so I guess you could say it wasn't actually "stolen" at all!
The reasons that more scooters (and strollers, and wheelchairs, and other stuff) isn't stolen at WDW are actually kind of simple when you stop and think about it for a bit. I think the first and biggest deterrent is the cost of a ticket; anyone looking to steal something to resell for a quick buck has plenty of other places around Orlando to steal from that don't cost in excess of $100 per day to get into. Next is the fact that Disney has a heavy security presence at the front of every Park (as well as at Disney Springs) and that probably keeps away a certain sort of petty thief as well. They want to do their business with the least possible chance of getting caught. Last, but certainly not least is the fact that Disney has security cameras (and other security measures) in place to deter and deal with Guest issues.
If you are really concerned, go online and buy either a Tile or an AirTag, (which one is "better" is a personal decision for the user, and is another topic for another post somewhere else) and hide it somewhere on the rental scooter. You can then check on it with your phone, and if it happens to "move" (a CM moves it around to help create/keep pathways clear) you will be able to locate it yourself quicker. Or, you can go lo-tech, and as I have recommended for years and years... you can just tie a brightly colored length of ribbon on it in a visible location (like the handlebars, or the basket) or a brightly colored bandana.
Don't worry! And have fun!