For me, I believe the sequence goes as follows.
Dani disappears.
LE gets information during their investigation that points to FG as being one of, if not the last, person to be seen with her.
They bring him in for questioning and he refuses to cooperate.
LE can get subpoenas and warrants for several things I'm sure while officially categorizing Dani as missing, but to get a search warrant for FGs residence, I would believe they changed her to officially to being a victim of a crime. I don't think you could get a warrant to search someone's home without there being a victim of a crime. My supposition.
LE executes their warrants, looking for evidence of Dani having been in the Oxford home. Looking for physical and/or forensic items.
They test the mattress, is positive for bodily fluids/blood. They take it.
They get a similar hit on the floorboards. Take them. We don't know how big an area they took. Only evidence we know if them taking from inside the home (that I've read).
I believe it's possibly the mattress where they obtained FGs DNA profile to match to Hines Park.
Now, testing the mattress, they would find at least two DNA profiles. The home owners. Samples that don't match Dani's. Hers is probably the only one they have. Remember, FG is no cooperating, so I'm presuming he didn't provide a DNA sample.
Same thing with the floorboards. They test.
We don't know the results of those tests.