The 300-Series does not have seat belt pre-tensioners in the SRS system.
The system has sensors mounted at the front, just under the air cleaner and battery tray (small yellow boxes). It is possible that these were disturbed, although they shouldn't have been.
Another possibility is that the garage disconnected the battery with the ignition still on, causing a 'Low Battery Voltage' fault to be logged ...
Either way, I'd take the thing back to the garage, complain, and have them cough up the cost of re-setting the SRS ECU, as this has to be done using either TestBook, Rovacom or similar. Disconnecting the battery doesn't work - I tried this on my P38 Rangie (very similar system), and even after 10 days, the fault was still there.
Good Luck!