The other option is that when they "fixed" the horn, they didn't leave the battery disconnected for long enough before taking the steering wheel apart. This means that there was still a residual signal going to the drivers airbag, so when it was put back together it showed up.
It might be a main dealer only code clear, so if you're interested in it, get them to pay for it to go to a main dealer to get the code cleared. I had something similar when I had stuff under the seat in my Volvo, hit a speed bump a bit quick and knocked the side airbag cable. This required a visit to the main dealer to get it cleared, as the local specialist couldn't do it on their system.
Hope that helps,