titletext shows up to all players, also hint boxes,
but you can try with the sidechat, you place a "dummy unit" that is on the same side of the player(s) you want to know the presence of the enemy units in a certain zone, when the trigger activates, it makes say the detection message to the "dummy unit", this is visible only for the side witch the "dummy unit" is on.
this was only the teoretical part, sorry that i cant help you more.