I've seen a lot of people travel. I'll go with the foreign dignitary theory…the secret service is responsible for protecting them when they travel in the US.

Cabinet officials that get that level of security (aside from the Kerry incident) don't travel commercially. Supreme court justices usually have 1-2 agents. Congressman and Senators usually have no security. Congressional leadership might have a couple people, but Capitol Police, usually in uniform.

Former presidents could have that level of entourage, but I'd be surprised for a former president not to be recognized.

It's a mystery