So I dug a little deeper and noticed that, at least on Mac, the first start of LO 3.5 (or later) migrates any existing user installation in ~/Library/Application Support/LibreOffice/3 onto itself.
This appears to be due to <http://cgit.freedesktop.org/libreoffice/core/commit/?id=6b522673373797bbf53d795d53e0ec45175a5d67> "default config location has changed, look in old config dir when migrating," committed towards LO 3.5, which added
Libreoffice 3=libreoffice/3to /org.openoffice.Setup/Migration/SupportedVersions/VersionIdentifiers. On a case-insensitive file system (as is common for Mac and Windows), this causes MigrationImpl::findInstallation (desktop/source/migration/migration.cxx) to identify ~/Library/Application Support/LibreOffice/3 (which would be used by both LO 3.4 and 3.5 just fine and without requiring any migration) as an old user installation at ~/Library/Application Support/libreoffice/3 that needs migration.
The migration happens only on the first start of LO 3.5, due to <http://cgit.freedesktop.org/libreoffice/core/commit/?id=995a87e5cf63fe1626245b62fef4aa71fa02dc94> "disable multiple migrations via MIGRATED stamp file." I do not understand what propblem exactly that is supposed to solve.
And would it not be better anyway to only migrate an old user installation if the new user installation does not yet exist? That should also fix the case-insensitivity problem on Mac and Windows leading to that dubious self-migration, I think.
Stephan