This is what I see from freeshell.org:
Wed Feb 04, 10:52:06  $ host -t soa gr.
gr has SOA record grdns.ics.forth.gr. hmaster-info.ics.forth.gr. 902040905 7200 1800 86400 1800
Wed Feb 04, 10:52:13  $ host -t soa gr. grdns.ics.forth.gr
Using domain server:
gr has SOA record grdns.ics.forth.gr. hmaster-info.ics.forth.gr. 902040704 7200 1800 86400 1800
Could this be the reason that we have a client not visible to the rest of the Internet? Maybe, since the secondaries seem to carry a version of the .GR zone with greater serial number than the master server.
Update: As usual, there are simpler answers: Like the customer forgetting to renew the domain name.