Ok I'll admit it I seem to have been outsmarted by Ben - and not sure how to stop him

Ben does a superb whistle recall, but a few weeks ago he started doing 'mock' runs. He suddenly took off as if he had seen something to chase, I whistled and he turned straight round and came back. I praised and rewarded etc, then about half an hour later he did the same again. I still hadn't caught on, so whistled him back, and rewarded etc. The next day he did it again a couple of times, he just takes off at top speed and goes well beyond the distance he normally goes away from us. Again perfect recall to the whistle. I started to notice that each time he did it he looked up at me before he ran off, and he also was running in a sort of an arc away from me not a straight line. So that I didn't end up constantly whistling and rewarding I tried to find another way to get him back without using the whistle. so I started playing with Harry and it worked, as he comes running back to see what I am doing and I can just praise him for coming back etc and reward with a small treat.
Yesterday he stepped it up a gear and I am stumped

He ran off quickly, I turned to Harry to start playing tuggy and Ben jumped over a wall in the field, which he never does. I panicked and whistled as I don't like not being able to see him, and of course he came straight back and got lots of yummy treats. A little while later he ran behind a line of trees, so while he wasn't very far away he was out of sight - so I whistled again. Today he did the wall trick 3 times

On one hand I am confident he will always come back, that is not a worry, and it reinforces his recall, but I don't want to have spend the whole time whistling and rewarding him, I want to just use the whistle when I want to - not when he wants me to

Any ideas please
