Recently, I noticed an odd behavior:
- some old jpeg files were modified for no apparent reason, and are now slightly smaller (some Kb)
- some xmp files were deleted
However, an explanation should be as follows:
- changes in jpeg files seem to be limited to exif data; I suppose that Lightroom does not check every image every time it's launched; the jpegs had their metadata upgraded (e.g. Adobe XMP Core version number raised to 5.2)
- the deleted xmp files were empty, and in fact reopening the same image in developer module forced Lightroom to redump another (almost identical) file.
Ultimately I guess that the reason for this tidying is that I opened my catalog with the original files online, but without the previews.