A laser that is powerful enough to hurt a human target (especially a human target with body armor) is going to be powerful enough that it'll be ionizing the air to some degree. It'll be like a lightning bolt, there'll be flashes of light and sharp cracking sounds. That's also ignoring the fact that the random bits of terrain that the laser is hitting will also be exploding. Someone under "suppressing fire" from a laser weapon would be quite aware of the fact.
PugJesus