How does the reduction of a ketone in sodium borohydride and alumina work?

I'm really struggling to understand my organic chemistry lab!!

In the lab, I reduced 4-methylacetophenone with sodium borohydride (NaBH4).  I was to find my own protocol online.  The protocol I chose was a microwave procedure in which the ketone was merely added to a alumina/sodium borohydride mixture.  The resulting product was an alcohol.

I am struggling to understand the mechanism of the reaction.  Unlike most similar reductions I am finding online, there was no water present in the reaction (just nabh4, the acetophenone, and dry alumina).  how does the reaction mechanism proceed, then?
What nucleophile attacks the carbon to release the OBH3- leaving group?

thank you!