You can pull the standby list in a variety of ways, one of which includes all kinds of info such as CLUB which is club member, VIP which is Very Important Passenger, and status codes (but you can also discern status from other ways too). OS1 = oversold awaiting seat assignment for an EXP, OS2 is PLT, and OS 3 is Gold. Same way for upgrades..UPG1 is EXP, UPG2 is PLT, and UPG 3 is Gold. I am not familiar with an OS5 code?

They'd use the CLUB indicator to know that you were a member and might be in the club if you were in a city where they have an AC. They might call the club if the flight is boarded and the club member is still missing thinking they might be in the club.

Did your club status get you on? I'm almost certain the answer is no. Perhaps the other people on the OS list ahead of you decided to take denied boarding comp?