I think that I read somewhere that the RAW-file modifications are stored in the RAW-file. The TIFF and JPEG modifications generate a XMP-file and that one can be stored seperately.
Am I wrong?
In Lightroom, modifications to RAW files are kept in the library or in a separate XMP file. If you use DNG format, everything an be contained in one file (RAW data, metadata and modifications). TIFF files modifications are saved into the same file.
You can do the test: in grid mode, select 1 RAW and one tiff or jpeg file. Make sure that you performed some changes in the tiff or jpeg file. Press CMS + S (on a Mac) to save metadata to file. Now, go to the folder with your two files and you should see that a XMP file has been created (same name as your RAW file) and the modification date of your tiff or jpeg file has changed.