scratch marks

[Pythagoras]

Obviously it's not, because the maths weren't there, but it looks a lot like a graphed power function...
I see what you mean about the tau rho. But as always with the Voynich, it resembles something but isn't actually one.
I don't think it's a pen test, simply because quills needed constant recutting and if this is an idle pen test, I'd expect to see more throughout the manuscript.
However, it appears to be done with the same ink as the nearby root.

indeed. perhaps it looks less like a Tau-rho and more like an anti-Tau-rho