This age old probelm of correct use of signalling continues to raise its head! Ged Wilmot wrote an excellent blog post on this topic way back in 2010, and he has just re-posted it as ADI's like myself keep on using it to help others! You will find Ged's 2010 post on signalling on Ged and Claire's new website. The fact that Ged chose to include this 'old' post on their new website is an indication of just how much this post is needed.
So, I hardly need say any more about it. What I will say is, having observed other instructors teaching learners, and heard the comments from the learners of 'Do I have to signal here?' followed by a reply which has indicated that the learner is taught specific occasions when they must signal, as opposed to having a discussion about how we give information to other road users, then it becomes clear that instructors need more help on this topic. Trainers in the first place clearly aren't helping here! Sorry if all this sounds harsh, even rude, but this is fundamental to getting the correct messages across to our learners by helping them to work out the solutions to their questions for themselves, helped by skilfully crafted questions from the instructor.
Ask questions you don't know the answer to, such as, "What made you decide you needed/didn't need to signal there?" - you might be surprised at the answer you get! Or, "How did you feel about the way that driver used/didn't use a signal?" Or "Were you sure about what that driver was going to do when he signalled?" Open questions which allow your learners to express themselves without feeling under threat, or believing they've done something wrong.
By developing the skill of helping your learners to explore their own feelings and beliefs, to reflect on what has just happened, to effectively 'teach themselves', you will produce young drivers who can think for themselves, who don't have to ask, "Should I signal here?".