Dogs can only sweat through their toes (which is why they can easily overheat, so you need to look after them in hot weather) and so they will not smell of sweat just because they have sweated and not washed. Where might your dog have been that they will have mixed with sweaty people? Have they been hanging out at the gym, perhaps? Gym use can be healthy and good for socialising, but extreme use can be dangerous. Watch out for changes in eating habits or signs of steroid abuse.
The other main source of sweaty smells is teenage boys. Has your dog been playing a lot of computer games, spending long periods alone or hiding porn in ultimately obvious places? They may be part of a group of teenage boys. Teenage boys are not harmful, but can be a little bit annoying, so just keep your eye on them.

Smell Synonyms: 
body odour
old socks
See Also: