I did this in my car by accident multiple times way back in the day (like 2004). Stop for food after work, eat the fries and forget about the burger, which gets buried under stuff (I keep my car generally cleaner these days; I was a teen). It dries out completely with no actual change in appearance, smell, nothing but turning rock hard. Gross.
Needless to say, I haven’t eaten there in almost 20 years, other than an occasional fries on a road trip when that’s all there is.