With the right ammunition and if a shooter takes adequate precautions, a .223 rifle would have little if any muzzle flash even without a flash suppressor. The flash only lasts a fraction of a second to begin with, so unless someone was looking right at him, they probably wouldn't see it anyways..
I took my 22 long rifle outside and discharged it with a 22 short in the chamber. I was surprised by the produced muzzle flash, granted it doesn't have a flash supressor, nor does it use any where near the gunpowder of the smallest .223 cartridge that I can think of. The flash is so bright and apparent, I believe it would attract attention, even if just glimpsed with peripheral vision.
I find it ironic that this shooting happened at a Home Depot, a large contributor to HCI.