A full-sized knife sheath, etc. would be too big for a P3AT mag. However, I have a cheapo Winchester mini multitool that is sized to carry on a keychain - about the size of a Leatherman Micra, etc. I don't remember buying it (maybe I did and forgot) so I think that maybe someone gave it to me. It has a small jump ring attached so it could go on a keychain and also came with a little, nylon holster. I don't know that I have ever even used the multitool as when I need a multitool I usually go with a full-sized one but the holster is just right for carrying a spare P3AT mag. With the flat floorplate instead of the 'pinky extension' floorplate, the top flap will even close fairly well (it is held closed with velcro). I only use it when carrying the P3AT on my belt (in a FOBUS) as the leather pocket holster I made has a place for a spare mag and the spare mag helps further disguise the pistol/holster in my pocket if it happens to print. As I carry something bigger as 'primary' when I can and don't use the P3AT as a BUG, I haven't used the little sheath all that much but even if I used it often I don't think it would be wearing out any time soon. I used to see these little Winchester tools at Walmart for about five bucks.
Speaking of 'urban camouflage', I can't carry a firearm while at work but I can carry pepper spray. Pocket knives and multitools are also okay. I use the nylon sheath from a full sized Gerber multitool to carry my spray on my belt while at work, hiding in plain sight (the tool got dropped in the pond while trying to get a hook out of a catfish's mouth.) I go old school and carry a Swiss Army type knife in my pocket in case I need a small screwdriver, bottle opener, etc.