When is it appropriate to salute indoors?

Conduct during the National Anthem is not in the Flag Code (Title 4 Chapter 1) rather it is in:
United States Code Title 36
§301. National anthem
a. Designation. — The composition consisting of the words and music known as the Star-Spangled Banner is the national anthem.
b. Conduct During Playing — During rendition of the national anthem —
1. when the flag is displayed —
A. individuals in uniform should give the military salute at the first note of the anthem and maintain that position until the last note;
B. members of the Armed Forces and veterans who are present but not in uniform may render the military salute in the manner provided for individuals in uniform; and
C. all other persons present should face the flag and stand at attention with their right hand over the heart, and men not in uniform, if applicable, should remove their headdress with their right hand and hold it at the left shoulder, the hand being over the heart; and
2. when the flag is not displayed, all present should face toward the music and act in the same manner they would if the flag were displayed.
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There is no distinction between indoors/outdoors.

For Navy ref: Navy Flag Regulations Navy NTP-13B
http://www.ushistory.org/betsy/images/ntp13b.pdf

Just an opinion but if I was in the audience and not a veteran, I would consider that a show of respect. As a veteran I know if he was to render a salute he would have to turn and face the Flag. But you all know about opinions

Saluting indoors is a no no.( USMC) Exceptions when under arms, such as Duty NCO, or pay call. Marine was indoors, and should not have been covered.

You stand and face the Flag and Salute for the Natioval Anthem. I don't care who your are. Stand up and Salute or Shut the Hell Up!

If covered, you salute. When in doubt, salute.

I got to be on the ice in my service uniform to be honored at a NHL hockey game: I’d have to track down the video, but I believe I was told just to stand at attention, just like when the anthem is played during military ceremonies indoors. I’m also pretty sure my hat was off. I agree that it is a pretty grey area.

When I was in the Army, The only times I saluted indoors was when I was reporting to a Commanding Officer, or for a promotion board.

Not knowing the full context of thos picture, one rule of thumb I’ve seen used is if the situation calls for you to wear headgear indoors as part of a formal event then you’re authorized to salute. This Marine would’ve been fine just standing at attention. Those criticizing this photo are simply demonstrating their ignorance.

The short answer I guess its the one I always hated hearing in the military... "Situation Dictates. "

When I was active in the Corps, there were a few instances in which we would be in public in uniform and were ordered to basically mimic the crowd of the public, to be politically correct. If the public were to bow their heads during a speakers prayer we were to do the same (although this is also naval tradition) I guess a more fitting example would be if everyone in the crowd was told to stand up and close their eyes (like for example during a blind poll, I'm sure you all have experienced this at least one in your life) we were expected to follow suit. Even if it was technically against naval or corps tradition.