If I play Grim Apothecary and my opponent reacts with Hidden Wages of Pain, does Grim Apothecary still trigger?

Yes, Grim Apothecary still triggers.

According to the rules of the chain and how units enter play:

1. The Sequence

• Playing the Unit: You play Grim Apothecary. It is a unit, so it enters the battlefield immediately (finalizing the action of playing it).
• The Trigger: Because Grim Apothecary has a "When you play me" ability, that ability is now placed on the chain.
• The Reaction: Your opponent reacts by flipping their hidden Wages of Pain. This is placed on top of the Grim Apothecary trigger on the chain.

2. Resolution (LIFO)

The chain resolves in LIFO (Last-In, First-Out) order:

1. Wages of Pain resolves: It deals 3 damage to the targeted unit.
2. Grim Apothecary's ability resolves: Even if the Wages of Pain happened to target and kill the Grim Apothecary, the ability has already triggered and is on the chain. As per Rule 359.3.e.12, if a unit is no longer on the board, information about its stats or status is "null," but the ability itself still resolves. You may proceed with the instruction to return a friendly unit to its owner's hand (if it is still a valid target).

Summary

Playing a unit and the subsequent "When you play me" trigger are distinct steps. The unit enters play before the reaction can be placed on the chain, ensuring the trigger is successfully added. The reaction from your opponent does not prevent your unit's ability from triggering or resolving.