Actually its a big no-no for agencies - this will be a plucky freelancer spending the afternoon looking over the fence. Its tricky work as clubs often threatens to ban said snappers from invitational training sessions and even games.
Even locals and residents in areas where training grounds back onto urban areas (Liverpool's Melwood, for instance) are regularly scorned by security. Sad but true.
Its a common thing for a lot of freelancers with a spare afternoon. Especially during transfer windows and when new people come in. I guess in this instance it really was pot luck.