The bike wasn't missing either. I took what Cory said as "Why would the first thing Mark noticed be a missing fishing pole, when he didn't look for the bike or footprints from Dylan?" The thought being, you walk into the house, your son is not there, you don't remember if the fishing pole was in the house or garage (Marks said that it could be in different places), but you are sure THAT is missing along with your son, yet you didn't look to see if his bike was missing, or look for footprints to see where your son has gone.
That is what I got out of Cory's statement. I have not read of any claim that Mark looked for footprints.