I was pondering this as well. IMO....they were definitely in tune with the fact that cell phones can be tracked. They both turned their personal phones off before they got to Bosma's house AND they turned off the BATE phone after they left on the test drive. So they knew the phone could lead LE to them. The question would then become, why not earlier?
My hunch is they killed Tim shortly after leaving the driveway, picked up the other vehicle and then didn't stop until they got to the Brantford area where they were able to get the phone off of Tim's person and discard it. The area near Kemira would be very dark at that time of night with pretty much no traffic. They may have even stopped in the driveway of the business to adjust their passenger. It would be interesting to see if the phone was up against the fence along the road or closer to the side of the driveway. I'm rambling now... MOO...completely theoretical at this point.
Up until the phone was switched off, and during the drive to the Kemira location, Tim's phone surely must have been going off from SB's calls and texts. We know the ringer was turned on from the testimony today that it kept "dinging" when the phone was turned on. Maybe this scared DM and MS and prompted them to dispose of the phone immediately and it was thrown out of the vehicle?