My gym has discontinued towel service because so many members were
just stealing the towels.  I'd like to know if there exists any kind
of detection system that could be attached to the towels, like some
kind of tag used at a department store, so when you pass between
sensors placed at the doorway, a signal would sound to alert the staff
somebody "forgot" to leave their towel in the bin.  The tag or
electronic unit would have to be waterproof and able to withstand
temperatures found in a laundry dryer.

How about making people check out towels?  They must leave a drivers
license which is returned only upon returning the towels.

No theft device is able to keep up with the heat. Yout towel will be damaged.

But some suggestions:

Take a refundable deposit of $10 - $50,  or a personal item (picture
id) in exchange at the time of towel issuance.  All-inclusive hotels
do this in Mexico. No towel returned, keep the money and buy a new
one.  You could get creative with the Id - Send the id to a refugee
camp.  Ok, ok.

Sell towels.

But if you are willing to do something with the towel to deter the
theft, put a deliberate hole in the towel (not too tacky one) and see
if anyone wants to keep the darn thing. Some cooffe shops put a hole
in their spoons so it makes it less attractive to take.

Eve