I don't recall LE saying they were no longer looking for anyone else, only that they didn't know if there was a third person or not. That's the last I recall of it ever being mentioned. This doesn't tell us whether they are no longer looking or if they just have no idea who it is or if he/she exists.
I guess this article written on May 27, 2013 is what I was referring to, which led me to believe LE had now believed they were mistaken about the 3rd person, but you are correct, they merely said the threat is gone, and not that there wasn't still necessarily a third person involved, which very well could have been a female driver of Millard's vehicle, a vehicle which LE seemed quite certain to have captured on video with a single driver. BBM in the quoted text below:
SUSPECT NO. 3
The only person Hamilton police are still hunting down in the Bosma case.
Not much is known about this suspect. Not even if it is a man or a woman.
Police have said Bosmas truck was followed by a second vehicle the night he was abducted. That vehicle was a dark blue Yukon SUV owned by Dellen Millard, according to police. They say only a driver was inside.
Police have not released a description of the driver, nor have they said what role they played in Bosmas murder.
Superintendent Dan Kinsella did cryptically say, after announcing the arrest of the second murder suspect, that, We believe the danger to the public has been arrested, and we believe the danger to the public has subsided as it relates to this case.
http://metronews.ca/news/hamilton/685845/the-tim-bosma-case-in-focus/