Running 'emerge --depclean' today I got this:
* In order to avoid breakage of link level dependencies, one or more
* packages will not be removed. This can be solved by rebuilding the
* packages that pulled them in.
* dev-libs/cyrus-sasl-2.1.26-r9 pulled in by:
* net-fs/samba-4.2.9 needs libsasl2.so.3
which, AFAIK, means that some net-fs/samba library or executable is linked to the libsasl2.so.3 library without it being a dependency of the net-fs/samba package.
If I 'emerge -avC cyrus-sasl' and the rebuild net-fs/samba, the latter is rebuilt without errors.