When a package is manually upgradeable, the user isn't informed in
general. There should be some config file to show such information
automatically, perhaps after the cache is updated (I'm not sure
how the output would fit in aptitude).

Note: This is important because the user could easily miss security
updates of experimental packages. For the specific case of Iceweasel
releases (which are provided in experimental), I've posted a message
there:

http://lists.alioth.debian.org/pipermail/pkg-mozilla-maintainers/2015-December/026994.html