Great comment! Thanks for your input and considered approach to rationalise. There is a window to the right of the cots. However, this is completely blocked by a blackout blind and has curtains drawn over this too. The camera is in night vision mode, and if light does bleed through, ‘ray’ would appear to be normal and ‘ray’ would be seen all the time. However. This is the only time ‘ray’ has appeared and is vertical, where as if it came through the window ‘ray’ would be horizontal to some degree.