Magritte cut it in 4 pieces and reused the canvas, not painting on the reverse side, but on top of the painted image, which I assume he didn't like. (It's not that good.)
The first 2 pieces were discovered in 2013, under "The Portrait" in the Museum of Modern Art and under "The Red Model" in the Moderna Museet in Stockholm.
Now, a third piece has been found, under "The Human Condition" at the Hamilton Kerr Institute at the University of Cambridge, whose historic art curator Dr. Giorgia Bottinelli say:
"If it was technically possible [to extract the hidden painting] it would certainly be unethical, as it was Magritte himself that decided to cut up one of his paintings and then create new compositions over the fragments.... The preservation of the artist's intention is in my opinion our main priority. Non-invasive and non-destructive imaging techniques will enable us to make a reconstructed image of the hidden painting."Certainly unethical?! I find that hard to believe. Does a principle of the "artist's intention" require us to look away from artwork he didn't choose to preserve for display? If so, why are we looking at the black-and-white photograph of it? How do we know he preferred "The Human Condition" (just because he hadn't cut it up or painted over it yet)? If we found a stash of Magritte's preparatory sketches in a portfolio labeled "Do not display. These sketches are to be burned" would we follow his instructions?