When you mouse over a tile, some tiles say 'noticed' and some say 'watched'. What is the difference?
Both these form part of guards' vision cones and will cause a different response if you agent enters them. Note that all cameras, drones and turrets (and some high level guards) only have watched tiles forming their vision cone, no noticed tiles.
Watched: Entering these will immediately cause the guard to see the agent, turn towards the agent, alert nearby guards and enter overwatch.
Cameras and unarmed drones cause the alarm level to increase since they cannot overwatch.
Guards or drones will remain alerted for the remainder of the mission.
Noticed: These tiles represent the guard's peripheral vision, entering these tiles will cause the guard to investigate that tile at the next opportunity; if they find nothing they return to their normal patrol route. This is the same mechanism as when they see a door open, hear a sound, get drawn in by a mission event, and so on.
- If this happens during your turn: the guard will not move until it is their turn, allowing you to "safely" enter these areas (just make sure you're hidden at the end of your turn).
- If this happens during their movement: they will immediately turn towards that tile, almost certainly causing the agent to enter the watched area and immediately alerting the guard.
- If this happens while the guard is in overwatch: they will ignore the agent in the noticed tiles, will not investigate and will continue to look (and aim) at their target.
Intentionally stepping into a noticed tile to pull guards out of position is a good tactic to use for manoeuvring stealthily. Note that guards will investigate the last thing they noticed, so you can use this trick to redirect guards that are investigating somewhere you don't want them to.
Of course, intentionally entering a watching tile can be useful too, but this is considerably more dangerous and you should ensure you can hide immediately.
To reuse an image from another of my answers (that is partially relevant), note that you see there is a visual distinction between these tile states. In the image, the watched tiles - those around the agent - are more strongly coloured, while the noticed tiles - those covering the furniture in the top-right - are dimmer, with more of the floor's colour visible.
If you're ever unclear of which tiles are which, remember to hold Alt to activate the tactical view.