Ah yes, the dreaded rotten eggs. Gas culprit is H2S, hydrogen sulfide.
With some quick research and with work knowledge I'll give ya 2 culprits. If your car is LPG, H2S is in it, though not in high amounts.
In some cases, dying lead batteries can produce the gas too.
So, you need to check for LPG leaks, exhaust leaks, and check the battery. Get it sorted ASAP though cause that sh*t is no bueno for your health.
And finally, yea dead animals can also produce the gas, so maybe @k31th is onto something to!