Ground beef smells slightly sweet?
Grass-fed beef does certainly smell different from corn-fed beef. I haven't noticed it smelling "sweeter", but it could smell gamey, earthy, or even sulfuric if the cow got in to some wild onions or garlic. And there's plenty of variation between different types of grass and even the same grass variety from season to season. 

Corn-fed beef is not going to have that because, well, there isn't much variation in animal feed corn.

Rotten meat has a distinctive smell and it's going to be the same smell in grass-fed beef.