Unlocking a ctOS tower will show all the mission objectives near them, including any investigation clues that may be in the area. To unlock the tower, all you need to do is get to the panel. This will usually require some door unlocking and camera hacking, as well as some platforming. Videos for each tower can be found below.
(Sorry folks, missed the map shot image. The tower is in the south central part of Parker Square - Southernmost of the two towers located there)