In the middle of a firefight, when a grenade is thrown at you, it wouldn't be natural to press a button to alert the comrades around you. In reality, you just try to get the hell out of there shouting: Grenade!
I think it would be more natural, if your avatar automatically shouted "grenade!" when it passes near the center of you screen, so just looking straight at it would trigger the warning.
I think it would be more natural, if your avatar automatically shouted "grenade!" when it passes near the center of you screen, so just looking straight at it would trigger the warning.