Vent pipe ? Ends above the drain

[froddan]

I tore out the drywall in a bathroom for a remodel and where the sink was there is what appears to be a vent pipe that extends from the drain, but it just ends open a couple of feet above the drain, in the middle of the wall.

This seems strange to me? Any idea of this could be something else, or is it a big mistake by the plumber?