If you can open a hole in the closet drywall you could poke a hole in the sole plate, through the floor, into the joist bay. Then, if you had one of those "extendo lights" (light on a skinny tube) you could poke in the hole, you might be able to see the top plate below. I certainly would not start poking holes in the first floor until I was sure what was on the other side.
If your mom is a real good sport you could punch an exploratory hole with a straightened coathanger wire, with a point ground on it, turned real slow to see where you are downstairs. Don't push it, let the tool do the work (to keep from blowing out a chunk of drywall). Have someone below holler out when it starts coming through and you may end up with an insignificant pin prick that is unnoticable or at least easy to hide. I have also seen tricks with magnet retrievers and iron filings or a compass to "see" through drywall. Hold the magnet against the drywall and go looking for it on the other side. A little compass will usually point right to it. Move it around to triangulate the spot. The iron filing trick can be spectacular but you need a real good magnet (we used rare earth magnets out of disk drives) and it can be a little dirty.
