There is a stain and an impression. The stain was reported by George and LE also; it sounds like the stain is very obvious. The impression is an outline left by the weight of an object/body on the trunk liner. Like if you left a heavy box in your trunk--that would leave an impression.
~BBM~
The stain was on the spare tire cover (pg. 19- "a suspected stain on the spare tire cover" and pg. 65- "sealed metal can containing a piece of stained spare tire cover").
The spare tire cover is differentiated from the liner on page 56 as evidential item numbers.
http://www.clickorlando.com/download/2009/0929/21147154.pdf