That's not a bad bag in the last post, very similar to what Marlboro used to offer in a fly fishing take down rig.
Discreet = doesn't shout Gun Bag, had an Assault Systems in the day for my HK91, everyone knew at a glance there wasn't a lever action in it. Same for a car bag, you have issues. First, if there is some Big Brand on it, that alone attracts attention, and attention attracts thieves. Choose carefully - something like Marlboro, Coleman, or other means low tier and so what. Dewalt, Makita mean $$ for drugs to a window breaker.
Discreet means not telling everyone you have really nice stuff. More like, Wallyworld grade get by nobody would be jealous about.
Franklin bag bag vs Demarini or Mizuno, Chicago Freight vs Millwaukee, Coleman vs North Face. You eliminate the cool factor, not make it worse.
Used is better than new. Look to flea markets, DAV, Goodwill, Salvation Army, not Tacticool Accessories R Us. Same with guitar or musical cases - hey dude, you got a Gibson in there?.
The problem with discreet bags is the ones selling on the internet all look the same - which means any crack head can figure it out. https://www.google.com/search?q=discreet+gun+case&es_sm=93&source=lnms&tbm=isch&sa=X&ei=ZlhWVLGyE5KhyATEh4LAAQ&ved=0CAkQ_AUoAg&biw=1024&bih=515
Also goes to looking like it belongs - tennis racquet case in a Mossy Oak Camo 3/4 ton diesel or snowboard case in August, not so much.
So the rules are: 1) go low vis 2) make it match the ride/lifestyle 3) keep it out of sight best you can. Junk on top means low priority.
Better to have it when you need it than feed it to a perp.