The bluff rules for sending secret messages imply you are attempting to do so through conversation, dropping covert hints into the conversation. However, I see no reason why you shouldn't be able to do it without words at all. I'm not sure that should necessarily be the bluff skill or that the person to whom you're sending the message should automatically understand without a check at all, but I can always figure all that out behind the scenes. Normally, I would not allow a player to take 10 because there is a risk associated with failure (incorrect information is imparted). However, the message being sent by Magyar is sufficiently simple and Colmarr is suffienciently smart that I think unleashed was correct in moving forward as he did. What you don't know, of course, is if Dietrich saw anything.