If my opponent has a unit at Rippers Pay, and hides their Pyke there, then use Pyke legend ability to return their unit to their hand, their hidden card goes to the trash correct?

Yes, that is correct. If your opponent recalls their only unit at a battlefield to their hand, they lose control of that battlefield, and any hidden card located there is sent to the trash.

Here is the breakdown of why this happens:

1. Battlefield Control

According to Rule 187.4.c, if a player has no units at a battlefield and the turn is in an Open state, they lose control of that battlefield in the following cleanup.

2. The Sequence

When your opponent activates a legend ability (such as the one used to recall their unit), the ability is placed on the chain.

1. Chain is Closed: Because the ability is on the chain, the game state is "Closed."
2. Resolution: When the ability resolves, the unit is recalled to their hand.
3. State becomes Open: Once the chain empties, the game state returns to "Open."
4. Cleanup: During the next cleanup, the game checks for battlefield control. Because the player now has no units at that battlefield, they lose control.

3. Hidden Cards and Control

As established in RiftJudge FAQ #10557, you must control a battlefield to have a hidden card there. If you lose control of that battlefield, any hidden card located there is immediately sent to the trash.

Summary: Because the unit was recalled, your opponent no longer has a unit to maintain control of the battlefield. Once the chain resolves and the game enters the cleanup phase, control is lost, and the hidden card is discarded.