This has been bugging me too, but I think I worked it out.
CCTV2 (the one nearest the bins) is on a rotate, 210 seconds to sweep right round if I remember correctly. We thought that meant it only covered the bins for 42 seconds so a getaway would be straight forward. Except there is private CCTV on Short B so apparently that's been ruled out, and the CCTV1 camera would catch him if he turned back the way he came in ie back towards the doorway and pub. Which leaves walking straight forward along Brentgovel St...
Now, I can't find the map to hand but I'm pretty sure there was a camera marked on either Buttermarket or High Baxter both of which meet Brentgovel, and there is also McD's in that direction. Perhaps, by process of elimination, they know he cannot feasibly have walked off in any of those directions without being filmed. It'[s the only reason I can think why they're being SO sure and why door knocks weren't done on Short B. I think they've known from early on that he didn't get past those bins.
Thank god Corrie wasn't wearing dark coloured clothing, it must have been much easier identifying where he wasn't