Mysterious file changes

I use rsync and .xmp files to keep multiple copies of the library accessible from Mac/Win.
Recently, I noticed an odd behavior:

1. some old jpeg files were modified for no apparent reason, and are now slightly smaller (some Kb)
2. some xmp files were deleted

However, an explanation should be as follows:

1. 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)
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.