"GM's Marriott code GMC is confirmed in FlyerTalk's publicly usable codes list alongside Ford (FRD) and Toyota (TOY). No specific verification-denial threads were found. General Motors has major hubs in Detroit/Metro Michigan, and Marriott properties near the Renaissance Center likely handle many genuine GM employees. US verification is rare — geography matters most, with hotels within 5 miles of GM's corporate HQ being the exception."

"I have never been asked (yet) at Marriott but usually I show up at the desk by handing over ID/credit card. Don't read this as me endorsing going crazy using the codes that you aren't supposed to use! Interesting about the client reports going back daily."

"FlyerTalk community specifically notes the MMM code as "the go-to code for the Minneapolis/St. Paul area, described as extremely reliable at local Courtyard and Residence Inn properties, with a low risk rating." One user documented using an IBM rate for 14 years at 80-100+ nights annually and "never once being asked for an ID." Another stayed 500+ nights and was never asked. The 3M code is regionally well-known near 3M's Minnesota HQ."