Solution
An Eye Witness Account
According to Brock’s eye witness account he had been hiding in the bathroom was able to describe the culprit and his methods because the open bathroom door gave him clear line of sight of the counter.
However, Brock also said he had turned off all the lights before leaving, and then gone back and turned on the bathroom light as he’d walked in.
If the shop was dark, and the bathroom was bright, and Brock was standing in the corner with a clear line of sight of the counter, then anyone at the counter would have had a clear line of sight of brightly illuminated Brock.
It was impossible that the unsavoury character with the gun could have been unaware of his presence, and so Brock’s story must be a lie.