Mani, this one is not ON exit, but NEAR exit.
Question is : Is it possible for axis to use exit , without getting damaged by that mortar ?
I think Axis player are damaged by that mortar. If that is the case a warn or PM is indeed correct, even when they only get like 50 damage.
What I would do in this, I go axis, go to exit, see IF I got Mortar damage, and than warn.
For me that is the most honest way to be 100 percent sure.
My advice : When doubt, move it a bit !