In this picture (link), I'm beginning to think they covered the floor with some type of powder, perhaps looking for blood. If you zoom it to full scale you can see that the white areas just don't look correct.
 
ETA: I found out that there is such a thing as luminol powder:
https://www.amazon.com/1g-Bottle-of-Luminol-Powder/dp/B00CX9CPWC/ref=pd_day0_328_1?_encoding=UTF8&psc=1&refRID=B1932EAA484AN82RHJD6

I have a theory that the DNA came from TH's pap smear slides; they were gathered by Factbender. This would make the conspiracy much wider. The way LE showed up in force for this March garage search suggests a large circle of conspirators.
 
I don't know if you saw it, but I just realized that the stains on the floor may not be visible because they are covered in white power: luminol power. If they did a luminol test and didn't share the results with the defense, that may be prosecutorial misconduct for withholding exculpatory evidence.