Let's put it this way -- if the DNA were from the 3 in this case, it must have been from the day or night before she was missing. And it must have been either borrowing the brush (which seems unlikely because the friends would have remembered Joran sleeping with Natalee in the hotel, no?), or what some of your minds are now thinking. Voluntary, though.
Actually, thinking aloud here, if Joran had gotten a separate ROOM at the hotel, she might have "been with" him the night before for a few hours in HIS hotel room. Remember that he said he was a tourist. Maybe she brought her cosmetic case into his room and since he had no brush, he used hers.
However, it might just be a friend from the trip borrowing her brush.
The toothbrush rumor was started by a lawyer for the liars. He isn't privy to anything info-period.
Think about what could happen to your toothbrush at your motel while you are away, if you leave it sitting on the sink.
A cleaning person comes in...male...he sneezes over the sink. A cleaning person comes in...female...she knocks your t.brush on the floor where there is still DNA from the last occupant of the room (male). Your roommate's loverboy comes in and--oh, no! he forgot to brush his teeth and he's going to make out. He grabs the first toothbrush he sees (yours) and uses it.
Etc. etc. etc.
Now I know why Mr. Monk cleans his toothbrush with boiling water before he uses it!