Well, that's odd, even for MC

[Mark VIII]

2019 update 2.

My solid just disappeared. Nowhere on the level list could I find a single entity. created another entity on a new level and that new level did not appear on the level list. Blank/hide was not used. The strangest thing was the solid was still listed on the solid's tree. On the level's manager I clicked "find level from geometry" and selected the solid from the solid tree. That did nothing. I finally created a duplicate from the solid in the solid's tree and a new solid appeared. Then I deleted the original from the solid's tree.

[Richard Thomas - Mastercam UK]

If you right click on a solid in the Solids Manager you can now choose Analyze.  Not an direct answer I'm afraid but at least it could tell you the level that Mcam *thought* the solid was on!

[gcode]

I have accidentally set visible level to "Range" in the Levels Manage

If you set Range to 1 to 100, anything on a Level above 100 will disappear

The same can happen if you use "Named" then copy something to an unnamed Level ... it will vanish

[Mark VIII]

No. Didn't have it set to range. The model was on the main viewsheet, which was visible.