Right, proof?
How could he show you this pudding? go into the agency and take a screen shot to send you?
Either you believe him or don't. If you believe he is a plant and ask him, then of course he will deny it. If he is being honest he will also say he is not a plant. Since this direct question gives you the same answer, then there is no point in asking the question.
Of course he has stated that he was a translator, and maybe the agency owners have more accesses then he did. Or since his time as a translator there has been other changes made. Also he has stated that he only worked at one agency and has no knowledge on the other agency's operations. Alls what I can say about suspicions is that when I was a security guard I tended to become more supicious and critical of any stranger. It is, after all, part of the job to have that mind set.