Lightroom does not write the edits to the XMP files by default. You have to specifically tell Lightroom to do so, and you probably didn't do that.
If you open one of the XMP files with a text editor, do you see your edits in there, or not?
I have the "automatically write changes to XMP" box ticked in the catalog settings (the first thing I did when I installed Lightroom in the hope it would save me if I ever need saving!), which I thought was all that was required. Is there something I've missed?
Please answer the question ... are the edits in the XMP files when you open them with a text editor?
Hmmm...it's not looking hopeful. Here's an example:
<x:xmpmeta xmlns:x="adobe:ns:meta/" x:xmptk="XMP Core 5.1.2">
It seems to refer to PhotoMechanic, which I used initially for culling/flagging, but since then I've edited them in Lightroom, but the metadata doesn't seem to reflect that.
No Lightroom edits in there, I would say the work you did is gone.
I think you could be right there.
Which begs the question, is this a bug? As far as I'm aware ticking the "automatically write changes to XMP" box should mean the file contains all LR edits, but in this case it doesn't. Seems to me it isn't working.
Question not aimed at you by the way, just throwing it out there in the hope someone from Adobe can look into it...not that it will help me.
Oh well, back to the edit I guess!
Thanks for your input.
As far as I'm aware ticking the "automatically write changes to XMP" box should mean the file contains all LR edits, but in this case it doesn't.
Normally, the XMP sidecar does contain the develop settings changed in Develop -- I just verified in LR CC 2015.10.1.
I don't know what happened in your case. Perhaps the option Automatically Write Changes Into XMP wasn't in fact set for that catalog or had somehow gotten reset. You can't verify this after the fact, unfortunately, since the setting is stored in the catalog, and the catalog got deleted.
Or perhaps there's an obscure incompatibility between Photo Mechanic and LR with respect to .xmp sidecars that caused LR not write the sidecars. Some incompatibilities in the past have occurred, but I don't recall these particular symptoms.
Going forward, you might try a culling a sample photo through Photo Mechanic, importing it into LR, editing it, and then seeing if the edits got written back to the sidecar.