I'm not an expert but I work around propane and today I filled out paperwork about propane, gasoline, etc..
Propane(LPG) and gasoline both contain benzene which can cause nausea, gastrointestinal diseases, kidney, liver, lung failure, and causes leukemia, skin diseases, and many other problems in small quantities of inhalation, contact to the skin, etc..
rotten onions are added for smell as a detection method, they used to use sulphur.
i greatly doubt the presence of propane, as it would cause an explosion when you ignite your grill..
but you should still be on the safe side of things, keep doors closed when you un-attach your propane canister from your grill and when you attach it, as small amounts of propane can escape.
make sure your propane canister has an OPD. overfill protection device. if your open/close valve is similar to a triangle, rather than gear shaped with several corners then you have an OPD.
also, make sure that the bleeder valve is closed to prevent leakage by turning it towards you when the hook up attachment is facing you. be aware that if you turn it the wrong way it will spit a little propane or vapor at you.
propane, gasoline, etc are all highly toxic in small doses, always wash hands, and protect your bird from contact.
also be aware that you can see propane in cold weather, but its less visible when its hot outside.
if you can smell it, its present. also, the more you are around it, the less you can smell it. exposure to propane will cause loss of smell.
also, look into HD5 propane at thrifty propane. most propane is considered slop garbage propane. you can buy propane that is cleaner and burns longer.