Per Alfred's statements and in my experience reaction to intercept does not occur unless the TF is on patrol.ORIGINAL: geofflambert
I think if you give your commander a reaction range and more targets are sighted, he'll go after them (perhaps) regardless of the "remain on station" order. That is to say, he will not automatically order a retire to home port if there are other targets out there. Nonetheless, using patrol orders is in most cases the best plan.
Having said that, Lokasenna just posted that he had a TF set to "Remain on Station" which reacted, and he is a very good observer/documenter of actual behavior of the game. I have no idea how this can be as I have not seen it in my games, but maybe the mod or patch he is playing differs from mine or there was some strange set of circumstances that brought it about.