When you find that a properly parked device has been moved, typically it's because they are continuing to shuffle strollers, wheelchairs, and ECVs to manage the size of the "parking lot".
This is one reason why I suggest people add something simple, inexpensive but
memorable to their
ECV - so that you can tell the CM "Hey, I need help getting the
scooter with the rainbow-stripe ribbons; it's been moved, and I can't walk that far". A CM is more likely to remember the rainbow-striped ribbons than the fact that they are looking for a Jazzy Pride Elite Mover-Scooter Deluxe with a basket, know what I mean?

My friend
@kaytieeldr puts a t-shirt over the seat back to decorate hers, but you can invest as a little as a dollar for a colorful bandana from Wal-Mart and "mark" your ECV for greater visibility.