Here’s a tip on how to find it.
Note down the name of the reference plane you are trying to find. You can get this from the instance properties of the object you are trying to move, under Constraints > Work Plane.
Create a new reference plane and name that reference plane exactly the same name as the reference plane you are trying to find. You will see the following message The name entered is already in use. Enter a unique name.
Expand the warning and look for the element I.D. Note this down.
The final step is to search for the element by its element I.D. You can do this under the modify tab, Element ID > Select by ID
Enter the I.D Number you noted down earlier, this will select the reference plane you are trying to find. Right click, and click Maximize 3D Extents
This will maximize the extent of the reference plane so it is visible in all views.