Got to agree about the lateral line being as detection, however surely to rub as aggressively as it does may be too much for sensing.
What if ... the tench was somehow 'marking' the rig with maybe some kind of glads along the line. Spawning fish rub up against each other in a similar fashion, maybe there is more to it than just getting into position to fertilise eggs ? Pheromones ? Danger pheromones?
Scary thoughts.
I've kind of known/suspected carp 'lie' on rigs to test them, but this sometimes looked like a fish recognised the bolt rig and rested its 'chin' where the line should be. If the bait is held down when it sucks it knows all is not right.
More scary thoughts!