Found in Version:
On the top exterior floor of Dreynim spa (the one with the hot pool), there is a piece of Mournhold set railing that you can fall through if you jump on it. It's ID is t_de_setind_x_dwall_01. For some reason, only the eastern railing does that, even though the railings on the other two sides are exactly the same piece. See screenshot; the piece in question is under the crosshair. I am using OpenMW nightly 20200219, TR v. 19.12, and T_D v. 7.1 (low-res).