The samething just happened to me. The sensor just came off. Today was a super humid day. Reading all of post about this issue I'm fairly disappointed in how Samsung is handling this. To be told water damage on a watch that's water resistant is the most absurd thing I've ever heard. On top of that the watch is about 9 months old.
This is what my watch looked like the other day when I woke up (see below). The back plastic cover had slided off during my sleep (yes it is a very thin sheet of plastic/gorilla glass). I'm running with the watch and it clearly looks like the glue has been dissolved by sweat. So beware that this watch is clearly not sweat resistant! This is a big dissapointment for me as I bought it 10 monts ago and payed 450 USD for it in Denmark where I live. Now I'm waiting to get it back from repair, but I don't have high expectations as I'm pretty sure the fix will only be temporary, as new glue will surely be dissolved again.
For me this is dissapointmen no. 3 with Samsung. First I was not able to download a new Android version to my Galaxy S8, just because I bought it in Germany (neighbor country to Denmark), so I realized that this is how Samsung control the prices in Europe (or maybe globally), very clever. I was not aware of this because I used Apple products before I switched to Samsung. So not able to get the new android version and unwilling to jailbreak it and loose warranty, I eventually gave up and purchased a new Note 9 and today I'm using a Galaxy s10 (purchased in Denmark of course, which is one of the most expensive countries in the world, the S10 was over 1000 USD here when I got it).
Secondly I recently noticed that my Tab S3 has been bend slighty. I was shocked to discover this because I have really been nursing my tablet and always putting it in my incase case when traveling. So the construction with glass on both the back and front must be really really week. The sad part is that I bought it in Best Buy in the US, so it is impossible for me to return it to the vendor as we don't have Best Buy in Denmark.
And now after this third dissapointment with my watch I must admit that I'm now looking for another brand, preferably one of the cheaper chinese brands as I've been using way too much money on Samsung products. It is really a big dissapointment as I love Samsungs design and own a lot of their products, even my TV is from Samsung. But I can't keep on supporting them when I see signs of lower quality and their prices keeps on going up and up. It's a shame :-(
Mine just feel off. I couldn't believe it. They put glue in the most humid place, but most shocking is that they said it was waterproof. I like Samsung but please could you repair the watch or switch. We pay good money to not have glue on our skin for months straight.
I have the same issue as everyone else with my cover sliding off. I also have never used swimming, in shower or other wet activities that it is rated for. I know that the heart monitor will not work without it despite customer support's claim that it is "only cosmetic damage". I had the same thing happen to my Samsung Gearfit 2 and replaced it with this expensive problem riddled item. Has anyone received actual assistance by private messaging their info as requested by the moderator? To add insult to the injury of this expensive non-operational garbage - the reply I received is that they cannot help me because I am in the very foreign far away country of Canada, where I should contact support even though they know "that it is limited". a very polite description of the useless support already received by phone. Will never get another Samsung product and that is after loyal customer of TV, dishwasher, stove, fridge, speaker, 2 tablets, 3 smart watches and 5 generations of cell phones!
Just had the sensor fall off on mine. Been using it religiously for about the same time. Same problem as the OP.
I've taken it swimming and done everything within the parameters the watch is designed for. It's been working great but now I'm going to keep it out of the water...
I pushed the cover back on so we'll see if it sticks