The Security Level is a mechanic in Persona 5.
The Security Level represents the alertness of the owner of their own Palace. It primarily increases when the player is discovered by any enemy or retreats from battle. When the meter reaches 100% it becomes impossible to explore the current dungeon and the player is forced to retreat to reality immediately. The only way to reduce it is perform an ambush attack and win that battle instead of retreating. Revisiting the Palace in the other day will reduce the meter a bit as well. The Security Level can be subsided by Ichiko Ohya's Confidant ability (theoretically Ichiko reports unfavorable scoop about the owner of the current Palace to distract that person from noticing the player's activities in the Palace.) If the player has connected to the online "Thieves Guild", a request can be made to reduce the Security Level when it hits 100%.
When the Phantom Thieves of Hearts send a calling card, the current dungeon's meter is fixated at 99% and will not boot the player even when being discovered and ambushed by any enemy.