I do appreciate the suggestions my readers gave me about Sadie's rolling in stinky stuff, and I figured I'd better address your questions and solutions.
"my best bet is to find it, double bag it, and get rid of it"
Maybe some kind of varmint poop - they do love possum and raccoon poop for rolling. You said it's not something dead, right?"
Maybe you should check out the place where she is wallowing."
It might be something dead. maybe you can get closer and find out?"
OK, here's the problem with "finding it and getting rid of it": We have 43 acres. The part where we walk is at the edge of wooded areas. When I turn Sadie loose, she totally disappears at times, and that's when she gets stinky. If I see her rolling nearby, all I have to do is shout "No!". But she isn't usually in sight when she does her thing.
As to what the smell comes from, I don't have a clue. If it's some sort of poop, it sure does have a different odor than any kind with which I'm familiar. And since I've had many varieties of livestock, I've smelled a lot of varieties of poop! If it's something dead, it's the same deal: It doesn't smell like any carrion I've ever smelled.
A solution would be to keep Sadie on her leash. But that half-hour of freedom is something she lives for. She's a very active dog, and after being cooped up in the house with a couple of aging human beings all the time, I like giving her the opportunity to run free, even though there is the chance of her getting stinky, and even a slight risk of her running away and disappearing.
So I'll just hope for the best, and stock up on dog shampoo.
Please forgive my making such a "crappy" entry. (And for attempting to make such a lame joke, but what else are you going to do with poop?)