I have never heard of the flashes being used to convey any meaning. Now the sgian dubh does. If you can see the knife on the other person, then that is his way of saying that he trusts the company that he is in. If you see a scot, and you do not see his knife, then he might be a bit jumpy.

Remember in the film Rob Roy? Liam Nesson's character kept the blade hidden in his tam.

Oh, and "sgian dubh" is actually pronounced: /skane doov/; in Gaelic, "bh" and "mh" are given the "v" sound.

-ian