Why is there a rotten egg smell near my propane tank?

I have a 500 Gal propane tank, we only use it during the winter for heat. It is currently below 5%. There is this nasty smell coming from near the tank and when you lift the hood (think that's what it's called) it smells really bad! We have the propane turned off right now since it's not in use so why would it smell like this?! Is it dangerous? Could there be a leak or is this normal? No smell once you get about 10-15 ft away from it.