First of all, I must stress that I'm mixing.
My accept_keywords list is large, though I'm not sure how large exactly, due to dozens of stale entries (for packages that not only went stable, but even long left the tree).
Anyway, today, at treestamp Tue, 28 Jul 2015 21:15:01 +0000, on two separate setups, I've noted a keywording request at the end of 'emerge -upvD -with-bdeps y @world'.
=dev-libs/openssl-1.0.2d ~x86 (on the other machine ~amd64)
However, the packages packages printed as the reason were bogus - neither had '>=openssl-1.0.2' dep. So, I masked 1.0.2* and added '--autounmask-keep-masks y'.
Only then was the real reason printed - it was www-servers/apache-2.4.12-r1 (one of accept_keywords entries).
Another variant of that bogus boost mask or a new problem ?
*** This bug has been marked as a duplicate of bug 419381 ***
Use the enalyze command to create a new keywords file.
# enalyze rebuild keywords
Diffing the newly generated file with the existing will tell you the stale entries. (provided you sort the existing file first)