I have tried ShowSelected, unhid all the layers, unlock, and other methods.
Try SelClippingPlane and SelClippingPlaneInViewport
For the clipping plane widget to be selectable it cannot be hidden, and the layer containing it must be active.
This maybe old (2014) but it still works. Locates clipping plane whether their hidden or the layer is turned off and reports back telling you what layer to find them.
One of Helvetosaur’s
Wow, that is really old, I forgot I even made that one…
Here is a (perhaps) more useful and modern version with Python… Let me know if it needs adjusting.
ReportHiddenLockedClipPlanes.py (1.9 KB)