I do not personally have any codes but did run into this problem and was told that they are getting very strict.
We booked our stay with a discount code that my sister got through work and
when we checked in they wanted proof--thankfully my sister was with us and had to show her business card and ID to prove she was who was on the business card so we were able to get the discount. Even if I had the business card, since I would not have her ID, we would not have gotten the discount. The clerk checking us in said that they have really gotten strict with discount codes that are attached to businesses.
I do not think that they can be strict for a general code as it is available to anyone who has it but with a specific code from a business etc they are getting strict. My sister got us this as an anniversary gift and decided at the last minute to come, which turned out to be a good thing.
So I guess the moral of this story is, get a general discount code and if you are using a business code, make sure you have ID--they wanted either a business card, pay stub with other picture ID (driver's license) or company issued pictured ID.
Good luck