When DD did her management training with M&S I am sure she told me that shoplifters had to be challenged leaving the premises and I think certain criteria had to be met, including opportunity, intent, visual confirmation and possession.
I suppose CCTV might have done away with the “eyeballing” but what possible evidence could they have if they cannot prove you have the offending item? Fair enough you might have picked something up, even taken it to a window or door for better light, but you could just as easily have then replaced it.
We’re you actually IN the shop in question on the day and at the time they say?
It sounds to me as if whoever is on fairly shaky ground.
I would go in all guns blazing (and frankly I don’t see why you should have to report to a police station anyway.)