As to the OP's question, by the rules the player is not allowed to decide if an area is GUR. If it wasn't marked then no relief is allowed under the Rules of Golf. If you weren't playing for anything except bragging rights with your buddies, then you probably could have ruled it as GUR and taken relief to the nearest point not closer to the hole where the abnormal ground did not interfere with your line of putt. If you want to get technical, a group of golfers playing only for their own pleasure and not part of a competition is its own committee, so they could rule on such an issue and still be within the rules. That doesn't mean that they can do anything they want and still be ok. There are very specific guidelines as to what the committee is allowed to do. Declaring an area to be ground under repair is one of the rights of the committee.