I though that clips were turned off unless you set the record mode to comping. My bad, but perhaps an opportunity for me to make a shut off switch for lanes into a feature request. I hate lanes with a passion because I find they do the strangest things when dragging clips around the time line. I did check the take lanes when one clip made another clip disappear. The MIA clip didn't move to a take lane.

ampfixer
The MIA clip didn't move to a take lane.

 I should have mentioned that when dragging clips around yourself, it's possible to have a MIDI clip get slip-edited into oblivion if you drop a clip on top of an existing one in the same take lane; immediate Undo is the only way to recover the MIA clip in this case. But I've never encountered a case where SONAR did this without assistance from yours truly.