Another time to turn off the Park Assist is when driving in heavy
snow/sleet. Packs up on the front sensors and will chime
incessantly until you get out and brush off the bumper.
Mute sounds great but not if only while stopped.

Important to note: while the green "P" icon tells you that it
is on, nothing reminds you that it is off. Perhaps Lexus could
add a yellow "P off". If you recall the Adaptive Front Lighting System
displayed a yellow "AFS" light when defeated and nothing when on.

 

Another time to mute/turn off the Park Assist is when...

You are carrying bicycles or snowboards via the receiver hitch and you put the car in reverse.