It's quite simple really - as you created the artwork you own the copyright, with or without your mark. I imagine the agency has simply not told his client he is using you and doesn't want hisclient to know he is outsourcing. However I would stress that you get some t&c's drawn up as quickly as possible.
If things ever got to a point where you needed to go to court then things like emails to and from your client discussing the work, versions of your logos, notes from phone calls, sketches all help to prove your ownership of the artwork. Again, the best way to protect yourself is getting your terms and conditions in place. These can simply be placed at the end of your estimates and invoices.
