The first time you use your MB, and the finger scan, it will take about 3 mins..pretty much the same as any other time. As far as making it to your first FP+ return time? Don't forget, there is a window for return, so you have an hour to use it. No need to worry about being on time.
The other thing....if, for some reason, the MB was misplaced, it really can't be used by someone else. First, the finger scan needs to happen. Second, if in a shop, a pin code is needed before the transaction can be completed.
I would imagine that if a MB was to be misplaced, you would notice it fairly quickly and get a new one. They would deactivate the misplaced one.