Exactly, just being exposed to any environment, it's going to pick up something. Dna evidence is really bullshit, it doesn't prove anything other than the dna is on what they tested, even that could be wrong, it's not 100 percent. Imagine how much dna is on a box of cereal at the grocery store for example.. just because someone touched that box at the store doesn't mean they were at a crime scene, the dna just shows it is there, it doesn't prove how it hot there.