Sorry, I've got stage fright!
Searchlights are on the roofs of most prisons. During lights out (at night), they can be seen as circles of light moving back and forth on the ground. If you are seen inside one of the circles with an Inmate Outfit on, a lockdown will ensue, and you will be sent to solitary (In TE1 you will instantly be sent to solitary, without a lockdown). Because of this, Guard Outfits, Infirmary Overalls or evasion are necessary for nighttime perimeter breakouts.