When is it appropriate to salute indoors?

SSG (Join to see) the regulation refers you to FM 3-21.5 which is now TC 3-21.5 Appendix A covers when to salute, including indoors occasions. TC 3-21.5 is available on APD.

Pledge of Allegiance confuses civilians when we are in uniform.

AR 600-25 para 1–12. Pledge of Allegiance to the flag
Soldiers may recite the Pledge of Allegiance as noted below in accordance with Section 4, Title 4, United States Code (4 USC 4)—
a. During military ceremonies, soldiers will not recite the Pledge of Allegiance.
b. At official functions, social events, and sporting events, soldiers should—
(1) When in uniform, outdoors, stand at attention, remain silent, face the flag, and render the hand salute.
(2) When in uniform, indoors, stand at attention, remain silent, and face the flag.
(3) When in civilian attire, stand at attention, face the flag with the right hand over the heart and recite the Pledge of Allegiance. Headgear should be removed with the right hand and held over the left shoulder, the hand being over the heart.

There are just so many variables, was the Marine in question active, or was he out and wearing the uniform for an event he was recognized at for being a veteran, and as such maybe confused as to what wheather a salute was proper? Covered inside while not under arms makes me believe this was orchestrated for appearances. Technically if he had been indoors under cover and on duty or armed he should have rendered a hand salute. Like I said this looks to me like a Marine Corps veteran at an event.

Maybe he was just adjusting his ribbons.

https://www.va.gov/opa/pressrel/pressrelease.cfm?id=1609

FYI for those veterans and service members out of uniform during the National Anthem.

I love that we have the luxury of spending so much time on a trivial issue like this. I believe the Marine is "showing" the crowd what to do. I now salute at baseball games and public gatherings in civi's. I was at Game 2 of the World Series and saluted during the national anthem and saw a kid a couple of seats away change from a hand over the heart to a salute it was cute. I bet he will be a future member of the armed services. Carry On

So correct me if I’m wrong, but a service member in uniform indoors in a lobby area of a sports game should move to the position of attention facing the general area of the music/flag and salute even without wearing any headgear?

At least in the Army, if you are indoors, a salute is authorized in uniform during the National Anthem. However, outside of that, it remains to be only when reporting to a senior or to a board.

He should've just been standing at attention but I guess we do what we want theses days thank his he didn't salute he would've never lived that shit down

The sergeant appears to be wearing a duty belt; hence, a salute would be appropriate.

To give an example; during the Marine Corps Ball ceremony held indoors, the commanding officer has the option of wearing a cover. If he/she does so, a salute is rendered to the colors presented to him/her. If no cover, no salute.

S/F