What can be asserted without evidence can be dismissed without evidence.
Then that's most of what you post BUT, it's quite simple David, an old sash window is made up of two frames; the top frame is in front of the bottom Frame. The locks sit on top of the lower sash and the bottom of the upper sash. There are various locks that were used (examples below). The screw type is basically just a pivot screw fitted to the bottom of the top frame. The catch (screw part) pivots back and forth and can be secured using the grove fitting which is fixed onto the top of the lower frame. To open it, you would simply make sure that it isn't screwed tighly. All you would need to do to open the window, is use something like a errrr a 'hacksaw' blade pushed up between the two frames and knock the catch out. To lock it (or give the impression that it is secured) is loop a piece of string around the pivot screw open the bottom sash keeping hold of the string, climb out of the window, close the sash and pull the strng free. You may even be able to bang this lock into place but the lock would be very loose; using the string, it can be made tighter because the string will create extra force to pull it into the groove. It works with a similar catch - see below, which is similar to the one I used and you would never know it had been locked from outside.
The window at WHF was old and probably a ittle loose, making it easier to perform the above. I'm not sure why you want to dismiss the posibility! I've tried it, I KNOW it's possible - you can buy the locks and make a mock frame and string is cheap as chips! You could even try it in the dark - but be careful though - you may need a net!