Thanks for the comments. There is certainly a voyeuristic feel to many of these which I find that quite interesting.

okmqaz42 - when I go on one of these sessions I lock my 17-50 lens at 17mm and focus to the hyperfocal distance (3m) and turn off AF, then keep the camera in TAv mode (I don't know what other brands call this but in Pentax world this is an ISO-priority mode where I select the shutter speed and aperture and the cam chooses an ISO). So my 17mm lens focussed at 3m at f5.6 gives me a depth of field from about 1.5m to infinity. So I just stroll around and press the shutter, usually without stopping (which is why I want to maintain a high shutter speed, I like to shoot at around 1/500s for these to try and minimise blur).