Enemy AI stand still not alerted by your presence yet.

You throw frag/smoke grenade nearby them, they don't react before or even after the grenade explode/spread smoke. They just stand still there, not scouting why a grenade landed nearby them or why did they hear an explosion or see smoke nearby.
But they react to grenade from Grenadier weapons (it even react with the sound when the grenade is propelled before even it explode) and bullets of course.

I have to add, that they move "out of the range" of the explosion if the offensive grenade is not "too far away", but they will do nothing else except return to their original position after a short while. Still, they don't really react to "why a grenade landed near me !?".