If an enemy Spotter spots the player, they will eventually trigger a District-wide Alarm.
Enemy vehicles under a district alarm will activate their Shielding and Panic Walls will raise, slowing progress for the player.
There is a period of a few seconds between a spotter initially spotting the player, and the issue of the district alarm. The player should either dispose of the spotter, or destroy the Comms Towers responsible for hastening the process.