Can physical object hiding be quantified with entropy?
If you can quantify it with a probability distribution, yes—any probability distribution has an entropy.

Here's an analogy: Suppose a line of 256 people each flip a coin, and put it in their left hand if it comes up heads or their right hand if it comes up tails; then they close both hands in a fist.  If you can't watch them flipping and placing the coin in their hands, you can't tell which hand each person has the coin in.  The probability distribution on your state of knowledge about where the coins are has 256 bits of entropy.

The fact that it's people playing the which-hand game instead of a CPU storing a 256-bit key in a register in silicon is immaterial; entropy is a property of the probability distribution characterizing your state of knowledge.