As a mathematics teacher, I have lost a number of calculators from students walking out with them. I have learned over the years that it is best to keep track of them by labeling them, as well as assigning one to each student. If I am missing one after a particular period, I can pin point that one student who was using it. I also keep this list posted somewhere near the calculators.
As you can see from the preview, I have mine color coordinated according to periods and subject matter.
Hope you find this as useful as I do!