B1
A hidden thing or place is not easy to find.
Dictionary examples:
My hidden talent for singing is best kept hidden, trust me! (0.0)
The hidden costs of my 'free' phone were so hidden, my wallet cried. (0.0)
The hidden button on my new phone stayed hidden until I sat on it. (0.0)
a hidden valley (20.4)
There were hidden microphones in the room to record their conversation. (38.7)