Phoo!
I know the smell you mean... and I think it's electrolyte!
It smells of fishy or rotting flies, not sure why. Electrolyte may be an organic compound or it may just contain a lot of sulphur.
Don't know whether it's a leaked capacitor or battery, but it goes after a while, maybe a few months.
I did hear of one guy who kept his Amiga tower open for a while and one day he opened it after a bad smell and found a cooked mouse inside.
Mmmmm...