While I have never been asked for a coupon on return, I have seen that message. For example, if you are using the BJ's discount, they have PDF coupons on their website that use to say coupon must be presented or something like that. They no longer say that, they just list the PC and DCP code on the pdf (of course, they only have one side of the pdf online, so you can't see the important terms on reverse )

Because discounts are usually applied when the car is returned, which I have always found to be very confusing as you really don't know your rate until you are done with the rental, that was when you may have been asked for the coupon, but not once has anybody ever asked. They just hand the receipt saying something like "your discount was here".

The only time I have ever been asked for a coupon (and I had it) was at an HLE location, as they couldn't figure out how the rate was so incredibly low. It was one of those freaky things you see here sometimes that you can get the rate down to a couple bucks a day if the stars were alinged right, etc.

Personally, I don't use any corporate employee discount I am not entitled to for the reasons above, should something happen there could be other things that go wrong like insurance, etc.