I've had runners cast Invisibility on 6 Grenades and a drone, and cast a heavily edged Manascape on one of the grenades and another on the drone, as well as a Stealth spell, just so those 10 spells allowed them to snipe a Master Shedim with two Semi-Automatic grenade bursts. Aaaah, the massacre.
Meanwhile, a Bug Queen spent like 5 Interrupt actions to dodge the KE IV grenades they kept throwing at her. XD
Back on topic: I only grant a free attack if they really earned it. In a situation where they suspect there might be attackers they can't see, you're right that there should be a defense test. Mind you, in that case there should still be a penalty for the defender as well. Maybe eliminating the cover bonus to their defense test, since they're no longer using the cover (asides from the Blind Fire penalty) to help make them harder to hit.
As for Detect Enemies: To quote a character's motto I played in a short campaign: I WANT people to know me and hate me. That way I can see them coming. (The character was, of course, a blind mage. Possession even, so he made his enemies turn on each other by various means of Mind Control. Which leads me to a potential third scenario: A possessed ally shooting you in the back.)