I work in a petrol station and we have a restricted sales log. Im not sure but i think its a legal requirement to keep one.
Each time we ID a customer for an age restricted product ( cigs, fuel, paracetamol etc ) we have to record it, weather id was provided or not. If id was not provided we just write the time date and what the product was along woth a brief description of the person. If ID was provided its the same thing but we say what id was provided and the name and DOB on the ID
Its nothing sinister, its there to act as proff that we are activly seeking ID from customers that do not look 25 ( or 16 for fuel ). If the store were to be done for failing a test purchase by the trading standards or police then we have evidence to prove we ask for it when requiired. It aslo used to show which staff members ask for id and which dont ask so often or maybe not at all.