unreal has tools for this type of situation, they are in the AI part of the engine
like seeing, hearing stuff like that. is more advanced so you need to invest some time in figuring how they work.
in your case if you have an " AI Sight" module implemented you can react when an object is seen.
or you can just use planned level design. so if you know that the player will come from a certain direction, when it is able to see the hidden object, add some triggers at that location when the player reach that point
maybe you are over engineering this 