So this is a new one, and my editing partner and I both ran into it within 24 hrs of each other:

Editing in Premiere Pro, and I suddenly realize the Motion tab is gone in the Source window > Effects Controls > Video Effects. You know, it's normally Motion, then Opacity, then Time Remapping, etc. I opened an old project, and they're still there. I tried copying/pasting Motion from another sequence, and it crashed (on my other editor's iMac, too). Bizarre.

It seems to be a sequence problem. If I open a new sequence in the same project and drop new clips in, the Motion tab is back. If I try to copy/paste clips from the problematic sequence, no luck.

I've searched for hot keys that I may have pressed – no luck.
I've trashed the Preferences and rebooted – no luck.
I've searched extensively in forums – no luck.

Has anyone run into this before?

Weird, motion pane only disappears for audio clips.

Has anyone run into this before?

There are a number of tabs in the Source panel, so many that they sometimes are out of view. There is a small bar at the top that you can slide to bring the desired tab into view. Perhaps that is your issue?