Mini II R56 (phase 1) airbag and seatbelt issue

I bought my first Mini (R56/2010) a month ago, and recently I’ve had an airbag and active seatbelt warning light come on.

I took it to a local garage, where they didn’t find anything serious and reset the fault codes. However, the warning light came back on an hour later. Do you have any ideas?

I noticed that other users with the same issues mentioned a plug under the seat, is that relevant?