This is the problem....
.....where was he heading ?
We don't have enough information to "guess" at that. But I don't think he was "up to something".
He was merely
"heading homewards". Whether that was to meet his mates first (and we don't know where they were) or to pop to his car (and we don't know where that was) or to go directly to the taxi rank.
Taking a look at BSE and their CCTV coverage, you can see there are "holes" in the coverage.
CCTV #1 is
"Camera 34" That's when you see him heading to the doorway.
CCTV #2 is
"Camera 9" And that's as we see him enter Short Brackland and go to the "loading bay".
Now there is also
"Camera 11" which is interesting, but I believe it didn't pick up anything of Corrie.
This camera is situated at "20-21a Butter Market". It's noted as "Dixon's" but I believe that is now Starbucks (?).
It appears to look in three ways (maybe more). I suspect it looks.....
A. Down to the Cornhill Shopping Centre (the alley between McDonalds and the museum).
B. Onto the Market Square itself and
C. Back down Butter Market, on to the oncoming traffic.
It is "probable" that this camera is also on an "automatic timed sweep" of those areas.
I don't know its "range" or "field of view".
If we have Corrie in Short Brackland, he would have to "cross the Market Square at the top" to head for the taxi rank OR "come out next to the museum, then cross the Market Square diagonally, to reach the taxi rank".
So "Camera 11" would be crucial to say "he was in the Market Square "
at some point" and "
in a certain part of it" OR importantly,
it could mean he never made it that far. But note, this camera is probably on an "automatic sweep", so those conclusions maybe not as concrete as they may first appear. However, if he crossed the square "diagonally", I would expect him to be picked up by this camera.
Reference The CCTV cameras
https://www.whatdotheyknow.com/request/319850/response/802518/attach/5/Bury St Edmunds.pdf
A rough location of "Camera 11"
https://goo.gl/maps/HVVWSr9SzGD2
Note, you can "click on the screen" and revolve around to see what "field of view" that position has of the "likely place Corrie would have been in....had he taken a number of possible routes (including "heading past the Cornhill Shopping Centre" if indeed he did so...albeit, he may have been missed by this "timed" camera).
So.... where his car "was parked", is quite crucial in understanding his decision making process.