We get them every Champions League game I think, it normally makes reference to the different entrances/seating areas in the packs that Lisa Eaton emails out.
I missed out on a ticket for Villarreal but had booked travel anyway (if only I'd booked with Thomas Cook eh!). I got a ticket in this separate allocation in the main stand through a friend of a friend of a friend who had hospitality. The ticket was in the main stand and I was sat with Mike Wedderburn, Kompany's dad, and 6 people wearing David Silva shirts amongst others.
Only time I remember these tickets being offered (at least publicly) to us peasants is Plzen where an extra 80ish tickets to add to the 600 general admission away tickets, but they were very expensive if memory serves.