If this is in a Post office I wouldn't be worried at all, if its a Box like in a apartment complex in the sun.. Sure A lil worry is fine but they should be fine even if left for hours. Yes Our cars get very hot and I have left my mod sitting in my car in the sun all day while at work.
your right lipos are even more dangerous. Any of these battery's can catch on fire, I was just stating that 18650s type normally don't catch fire as easy as a lipo does. I would rather you be over cautious then not to be worried enough.. Some people scare me with such battery's and how they treat them.. You should try your best not to drop any battery on the floor.
I have used Nicad, Nimh , Lipo, Li ion, battery's for years and years and only ever had a AA Catch on fire Lucky it mostly just smoked. it can happen and everyone should be safe. We don't want anyone to burn down their house. its happened many times with Lipos