I was asked for proof for the first time ever this week at the Las Vegas Marriott.
I laughed when asked because I was so taken aback that I've stopped carrying my company ID. Thankfully between my business card and showing her my email signature she was willing to accept it (why those two things were enough since they could be faked is a different story but wasn't going to complain at that point).
She told me it was because our rate was so good this week relative to the publicly available rate that she wanted to make sure that I was legit which is fair but still a first for me anywhere on the globe and not the first time our rate has been much better than publicly available rate