Well "Dave" and I went through this at h'row when they were doing a test run, and I was bloody livid. They didn't say what it was, what it was for, or the degree of um, clarity... I only realised when I was waiting a bit longer for DH to come out the other side and started talking to one of the customs women.
I was speechless at the time (has some uses then) but by teh time I was on the plane I was incandescent.
S'pose my real gripe is not being told/rights etc., but would not do it again.