Thanks for the info everyone.
I found the installation instructions online, and this unit does indeed have primary and secondary internal drain lines coming out of the bottom. The drain line is trapped and runs to the exterior.
But the instructions say it should be attached to studs or in a hanging configuration, and it is neither. It is just resting on what looks like to be sheet rock on a plywood board. The seller says this is ok by code, and the buyer doesn’t think it should be just sitting on the shelf. I advised the buyer that according to the installation instructions, it is not installed correctly. I haven’t heard back from either of them since I told them that.
And yes, that is a clothes dryer vent running up the wall from the laundary room that was next to this closet.