How do you find the timekeeper portals in Quell Memento?

When you navigate to a stage that you've already completed, on the screen where it shows all 4 levels at once in the window frame, it will display a brass ring next to any levels that contain a timekeeper portal.

To actually uncover the timekeeper portal, try and discover it in the same way as the hidden jewels. They are normally in the sneakiest place we could think of!

I found them to be excellent puzzles and noticeably harder than the normal levels.