First off - this is really a great item for end game. The point in time when you get it and how it works is basically awesome.

There are just two minor flaws (which go hand in hand) in my opinion that I've seen done differently in other Metroidvanias:
- Once a secret is uncovered, it should stay uncovered
- The notification sound should stop (so you don't unnecessarily search rooms for secrets that you've already found)

I think it is somewhat important to combine it with the secret staying revealed though because you might e.g. walk into a secret area, the sound stops but for whatever reason you don't open the chest or you don't follow the door.

Then you pass through the area at a later point and... you won't know that there's a secret you already discovered but didn't actually take advantage of.

Example might be: You don't have the secret detector, you accidentally slide a bit into a secret area but don't notice it, the game marks this area as discovered and when you return, it won't alert you.