There was bear spray residue on the hands; however, that does not mean it was sprayed at the bear or whether or not the wind direction could have blown it back into his face. As for speed of bears, yes, they are fast and one reason why next to impossible to hit while running; especially, directly at you with a handgun, and to hit where it will do the most harm (just behind the left leg in the chest...bears have a very strong breast bone that helps to protect the heart). Not saying it is impossible, just saying you need nerves of steel and be one heck of a shot with one heck of a bullet.
Bear spray, if the conditions are right does work; used it twice and I can attest it does work. However, like anything, conditions have to be in your favor or else you are just another flavor to the bear. Dogs help too, just keep them on a leash so they remain beside you. (Big dogs, Chihuahuas probably not so.)
When in bear areas, best to keep all your senses going. A moose carcass that close would most likely have smelled, unless it was an older carcass. And dead animal parts do have a very notable smell. Once you smell it, you won't forget it.
And old males can be quite cantankerous (just like us old Human males) and messing around our dinner can be a dangerous thing to do.
I do a lot of camping in that area; it's just west of Yellowstone NP (as well as just east of Yellowstone NP), and never seen a grizzly, but on the eastern side of the NP, I see grizzlies all the time.