Looking at my MQ2 Adventurer and at the Garmin Site Pictures, the top right watch button has a strong resemblance with a watch crown (hope the translation works).
Embedded into slightly risen side walls (not like any of the other three buttons), it even can be rotated - or at least the rubber "slips".
Screen shot from Garmin site:
Judging by the pics, all MQ2 instances have this distinctive pattern.
Playing around did not yield any noticeable watch reaction.
What did I miss?