When I played for Liverpool, fans could stand on ladders and bins to look over the wall and see us training at Melwood.
We were fairly certain that the odd 'supporter' would be working for our next opponents but, to be honest, it didn't really bother us.
There was nothing we could do about it and believed what they could learn from our sessions was marginal at best.
The Leeds story is entertaining but, from a professional's point of view, it's a mountain out of a molehill. Frank Lampard said himself that he couldn't blame Derby's defeat on having their training watched.
Teams can see how their opponents play every week either by scouting or watching video analysis.
The only area where you may get a heads-up is set-plays but even then you still need 11 players to be concentrated enough to carry out instructions. Spying on another team is not right and I don't agree with it at all but I'd never use it as an excuse for losing.
Watford and Arsenal's training grounds back onto each other so I don't know how you are going to stop them having a sneaky peek before their games!
I think a bigger concern for managers is players using friendships talking to other players about line-ups. That's the way we used to find out what the other team were doing and lining up.
I think that's been more influential than having someone hiding in the bushes looking at set-piece drills.