Guilty of bumping an old and barely coherent question I know, but I've had exactly the same issue with the parking sensor for the last few days on my 2007 IS250 and wasn't having any luck trying to find a solution including online.
(Normal operation being putting the transmission into reverse produces a "bleep" and activates the camera screen, the rear sensors then function accordingly, but the bleep and the sensors simply stopped working, with no fault message/code present).
So this morning I reversed it onto the drive in order to poke around, check fuses etc. As the drive is quite tight I pressed the button to retract the side mirrors...as soon as I pressed it the sensors immediately started working again 😅😅 😀
No idea if I'd accidentally reset something (don't think I did) or the ECU was confused or if it's just something random, but I wanted to put this here for posterity as a potential solution should anyone else come a searching. Have restested it several times and all seems well. Certainly beats an afternoon mucking about checking fuses etc 👍
PS - hi everyone!