| Well, sometimes they do that on the advice of BMW/MINI's technical department. If a problem can't be easily traced or reproduced, then they'll advise to reset the trouble code and see if it presents itself again (to rule out any intermittent or single-incident faults that might just be flukes).
Our dealer did this when our check engine light first came on and they weren't able to locate the source of the "super fine leak" the trouble code was reporting. When the light came on again less than an hour after leaving the service dep't, they did a more thorough investigation and found the crack in the charcoal canister. | 
