Assuming the garage is attached to the house......
Have they actually cemented over the door or have they taken the door off, bricked it up and cemented over that?
If so, they're probably now using the garage as an unofficial extension in which case it might be of interest to the planning dept but not the police.
Loads of people in S London used to do this- my friend's sitting room was the garage and it was kind of obvious that was the case from the one minute window(was a massive house share so they didnt mind that much)