Not sulfurous; nitrous. That's what nitric acid smells like.
Surprised you have the ammo in a hot garage. Board member Slamfire has a post somewhere on this board describing the Naval experiments with aging ammo, which they did in an oven at 140°F, IIRC. Took it about 18 months, but the deterrent was consumed in the powder, raising the burn rate and causing pressures to go from about 48,000 CUP to about 72,000 CUP.
When I was in a Gunsite class in the early '90s, the police lieutenant acting as range master told us his officers had reported ammo going bad in as little as one summer in the trunk of a car in the Arizona sun. That would be hotter than the Navy's oven was, though.