I always put it on my forehead until a wave washed it away and with a hood on I didn’t feel it leave so now it’s under the chin. With the price of mask these days you don’t want to loose them!
Not sure why it’s become taboo. In the early days it was taboo to wear the knife on the outside of the leg where it could catch the weight belt but now you see it all the time and I still think it could catch a weight pocket.
As for other divers watching for signs of stress, the more they know you the less they worry. I was once trying to bleed my tank down to 500psi to do a weight check while at a safety stop and had both regs freeflowing and out of my mouth to blow thru air and no one came over to check on me.