I place them in an archive folder and these actually go in a separate, not as accessible drawer. So I have a group of folder in the active physical reference section and a group of folders in the archive section.
I keep the same labeling conventions so it's easy to find them if I need them.
But I always first search in the active reference section and then go to the archive if I can't find what I'm looking for in the active reference section.
I must say that rarely, rarely happens.