the AR has a floating firing pin, like most shotguns. it CAN fire if dropped just the right way, allowing the firing pin to come forward hard enough. It does take a pretty darn good hit on a primer to ignite it. Chances are the small dimple you see on the primer got there from the movement of the firing pin as the bolt came forward and stopped. I doubt it was hard enough to be able to fire it though.
But honestly, you're best off not loading it in the house unless you intend to use it. I keep my AR with a full mag inserted, and my shotgun with the magazine loaded. But I don't keep either with one in the chamber. I know people who have dropped shotguns while hunting, safety on, one in the chamber and the gun went off.. thankfully pointed away from them and others. It has to hit just right for it to jolt the firing pin forward hard enough, but it is possible.