yes. Somehow sometimes that can happen.
Usually like Wolfden said, logging out and back in could help. Or adding it to the menu yourself.
You can try to run from the commandline: V
ox (watch the Capitals)
if command not found, simply reinstall the virtualbox-bin package, and then it should work and for sure appear in the menu.
(that's also explained on the VirtualBox Wiki page)https://wiki.sabayon.org/index.php?titl ... Virtualbox