Can I react to someone playing a hidden card, before it is on the battlefield?

Ruling: You cannot react to a card being hidden. Playing a hidden card involves first placing it facedown at a battlefield (which is not playing), then later flipping it faceup (which is playing the card).

Sequence:

• The card is placed facedown at a battlefield you control (this is not playing)
• Later, the card is flipped from facedown to faceup as a reaction (this is playing the card)

Nuances:

• Placing a hidden card facedown does not initiate a showdown open state, so you cannot react to the placement
• You cannot react to a card being hidden