The Super Mario games solve this really neat. Instead of a counter, each triangle (in Super Mario "star") has its own representation. This way you have an additional hint, where you missed the hidden triangles:
- ΔΔΔ - no triangles collected, yet.
 - ΔΔΔ - found first triangle.
 - ΔΔΔ - found second and last triangle, but missed the first one. I have to backtrack and search the first triangle at the beginning of the level. It must be located before the place, where I found the second triangle.
 - ΔΔΔ - I missed the second triangle, but I have a hint, where to look: It must be located after the place of the first triangle, and before the place of the last triangle.
 
I really like to explore levels and search for hidden stuff. With such additional hints, the secrets could be hidden even more, without frustrating the player.