Oh wow, sorry to hear but interesting to see the guts like that. I just encountered the other poster's sensor detachment thread last night. Clearly a failure of the adhesive or maybe the application of it. Surely they will want to track it back to see where or why this happened. Not good though.
To me some of the comments make it sound like Apple is the only one who uses adhesives. I'm assuming it's pretty much industry wide as things get smaller and smaller and harder to otherwise secure parts in place and avoid vibrations loosening connections. Does Samsung or LG do anything different to secure their sensors? I do wonder if the adhesive or bonding agent used for the sensors is specific to the type of metal for the different cases. Could be a simple problem of someone using the wrong specified agent. Well in any event Apple will undoubtedly make it right for the user and I'm sure they will be looking over these bonding failures carefully. Ah, the fun of being a first gen owner. Still I wouldn't trade my watch for anything right now.
OP keep us updated with how it's handled.