You can use two checks to achieve that, first one that checks if the player is close enough to any of the other characters by looping trough all of them every 5th frame or so, then if some are close enough make another check using a line trace from the player to that/those characters to ascertain if the player can see them, (the only reason for delaying the checks somewhat is to try and minimize any performance impact if you are checking against many characters/complicated terrain collision).
This type of checking is kinda basic since it ignore foliage or other types of partially covered characters, but it should work for you at the moment based on your image so I would recommend it.
You could also try and put up a system using ue4’s perception components but hat involves more options and can sometimes be more complicated to use cleanly.