Hi there everyone. I have noticed that the coolant in the reservoir keeps getting low so I have to keep adding. Usually when i drive for a long period of time it smells like a fart

and I can assure you that it's not man made

The car has never really overheated the temperature needle has only gone as far as to the middle. It's had this problem for quite some time now. There is no noticeable smoke that I have noticed. I'm suspecting a gasket but how can I be sure???? Thanks for looking.
-Gus.