My trunk is heating up?

This may be a VERY stupid question...but I noticed this evening that when I took a box out of my trunk it was very warm. So I checked under the mat in the trunk - next to the spare tire and the metal was hot to the touch.

What could be causing this - or is that normal???????

Thanks!

your muffler is under your trunk and its very insulated unless its extremely hot hours after i wouldnt worry about it