the exclamation point is the better idea, mainly due to the fact some buttons only appear in a single frame the disappear, im talking about the adele button, i missed it so many times because it only shows up at a single point in the dialogue so its extremely easy to miss, dropping hints is all well and good, but if u think of things in a different way you could come up with something great, you could create a screen effect that indicates perhaps panic during a scene indicating you have to do something (like a pulsating frame indicating that if you dont find something there will be a major plot change for example if you dont save adele something bad will happen), and to top that off you can have the text auto hide itself telling you to search the scene, this isnt telling you what to search for its saying hey wait lets check this shit out before moving on, as for basic hidden things like an extra scene or an easter egg the ! would be a good indicator not to skip just yet and look around, the ! isnt a give away its an alert, and people who say its not realistic arent thinking logically because everyone would have a different approach to a situation so a dialogue hint isnt the "realistic" choice either, like the kidnapper scene, if i was in the mc place i woulda knocked his knee out yeah, but id follow that up by driving my heel into the guys spine incapacitating him for good (cant do much if his spine is dislocated or broken) also the mc coulda thrown the bottle at the store and that would have brought the clerks running (hey if u smash the store window with the bottle that would get peoples attention), you dont need to go for the head to deal with him there are multiple ways of dealing with that type of situation, so having a single option with a vague hint that u need to do something doesnt really work in this sense if you get my meaning. but ya a physical indicator that there is something there is the way to go in a game like this, besides its a game not rl, so no matter what you do youll never make something that is "true to life" all you can get is "close to life".
this is just my 2 cents on the hidden button concept, love the concept didnt care for the execution, but like life everything is trial and error, but doing polls for major changes like this in the future will help out alot and avoid unnecessary confusion and frustration for people (i know i was frustrated with the adele scene i didnt even know about the buttons till i ran a search for the walkthrough the one i found didnt do any good and in fact made me even more confused, i literally just played the game for the first time, so i have no idea what ive missed prior to that scene), also id suggest putting in a pop up alert telling you that the hidden buttons are going to start appearing, seeing as i have no idea at what point in the game you started implementing them, hope it gives u some future ideas zee