When Marine and POW flag is lowered before the US flag, is there an obligation among soldiers to salute?

I am on an army post where some marines also work, and every evening they take down the marine flag, the POW, and the U.S. flag. The soldiers all stop and stand at attention while the first two flags are taken down, and then salute when the U.S. flag comes down. But a few soldiers I work with who are ex marines tell me that it is not customary to stop until the U.S. flag comes down. Does anyone know the military custom and courtesy on this? Can someone link to the reg? Thanks.