You are referring to the "in pectore" Cardinals. They are
revealed anytime it is safe for them. Some were revealed soon after
the Holy Father choosing them, others were never revealed.
Cardinals in present day China (2013) would be an example:
from Wikipedia:
In pectore (Latin for "in the breast/heart") is a term used in
the Catholic Church to refer to appointments to the College of
Cardinals by the Pope whose names are not publicly revealed (viz.,
reserved by the Pope "in his bosom"). This right of the pope is
rarely exercised, usually in circumstances where the pope wants to
make a statement for later historians about the honour due a
particular cleric, while not wanting to endanger that same cleric
in his present circumstances of persecution.