What happens if you cast Counterspell on a creature that is not casting a spell?

Inspired by this question.

Here’s the scenario:

A party member wants to fool an enemy into thinking that they are casting a spell. They take out components, do hand gestures, and starting chanting but do not actually initiate the casting of any spell. The enemy, fooled by this, casts counterspell at the pretend-spellcaster. How does this resolve?

Is counterspell successfully cast (obviously with no effect)? If so, is the slot used to cast counterspell lost?

Or, can counterspell not be initiated in the first place (since there is no real spell being cast)?