Originally Posted by ohmark

The OP doesn't explicitly say this, but I'm assuming that when he/she checked availability of the corporate rate AFTER check-in, he/she used the MARRIOTT website, not the corporate travel site. If the corporate rate showed bookable on the Marriott site after check in, it obviously should have been available at the front desk at the time of check in. I vote front desk incompetence/stupidity as the most likely culprit, with malice/thievery, sanctioned or unsanctioned by management, a distant second.