movie theater, we got only what is now the Royal Anthem, God Save The Queen. Came at the end of the evening. I recall everyone stayed and stood to hear it, including we Yanks. No patriotism on our part of course. Just common courtesy.
Same reason we JWs stand for anthems now. Even in countries where we're vilified for not signing up to kill the country's enemies. But I digress.
That anthem shared its tune with My Country, 'Tis of Thee, an American patriotic song. Its lyric phrase "of thee I sing" gave the title Of Thee We Sing to a musical by the Gershwins, "The musical lampoons American politics", says Wikipedia. Fake. Traitors. Sad.
Sorry. My hostile thumbs got carried away.
Back to your comment. Would there be some Americans who would hear the tune and think it was the anthem, and stand or kneel as a result?
Anyway, sometimes standing can be dangerous, as in the account of a patriotic ceremony mandated by a country's leader to glorify the state and himself as objects of worship. Daniel chapter 3.
The danger of a pose comes from its comparison with the mainstream, so you'll want to have a good reason for being different. There can be consequences; fiery furnace, collusion, whatever.