Good thing you sent a picture.....pictures always help.
I see an enclosed channel running along the bottom of the wall under the window. It runs from your sink and over to your tub enclosure.
Can you see anything under your sink which indicates what might be in this channel, because it heads under your seat in the tub, and this may be to permit a run of electrical, plumbing, or whatever.
Only you know what lies behind the back wall of the tub, and maybe you see things there too?
Ken and Terry
2010 Georgetown 330TS being pushed by our
2017 GMC Terrain toad (4 wheels down)