It would be more nice to store the cache database under, because when backup of the settings and certificates the whole cache is also stored:
See Apple specification: https://developer.apple.com/library/mac/documentation/FileManagement/Conceptual/FileSystemProgrammingGuide/MacOSXDirectories/MacOSXDirectories.html
$HOME/Library/Caches/net.syncthing.Syncthing/<database>
For systems who implement XDG Base Directory Specification it could be under:
$XDG_CACHE_HOME
If $XDG_CACHE_HOME is either not set or empty, a default equal to $HOME/.cache should be used.
See spec: http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html