Apple watch automatically inputting random numbers for passcode

The watch’s behavior looks like a brute force passcode attack. One wonders what it would do if it found the right passcode.

not much, and it isn’t what you thought.

watchOS 10.4 includes new features, improvements and bug fixes, including:

• Tap to Show Full Notification setting now allows you to double-tap to expand the notification
• Using Apple Pay with Confirm with AssistiveTouch will require a passcode for additional security and will not support double-clicking the side button
• Resolves an issue that causes some users to experience false touches on the display
• Fixes an issue that prevents contacts from syncing to Apple Watch for some users

For information on the security content of Apple software updates, please visit this website: Apple security releases - Apple Support

Hasn't happened since the time I put the watch in my pocket and when I took it out it was randomly putting in passcodes (until it finally locked itself out for 8 hours). I'm assuming updating the software fixed the problem BUT haven't put the watch in my pocket while walking so don't know.

It may do. Reset the watch like this

If you forgot your Apple Watch passcode - Apple Support

then set it up again, if necessary as a new watch.

If it does it again then it suggests a failing screen digitizer.

How did you solve the issue?

Never put your watch in your pocket or attempt to clean it without powering it down.