On Wikipedia is the claim "The telescope also originally had military intelligence uses, including locating Soviet radar installations by detecting their signals bouncing off the Moon."[0], which gives this[1](by name, but unlinked) as its source. It would be interesting to know more.

[0] https://en.wikipedia.org/wiki/Arecibo_Telescope

[1] https://www.youtube.com/watch?v=ZTC_RxWN_xo