I have my Ant Agent set to delete activities on the device after successfully uploading them.
I started this morning with no activities in the History of my device.
I ran 8 miles, and reset the device to save into History.
I reviewed my stats and laps on my watch from the History.
The data transferred and uploaded to GC just fine. It just wasn't anywhere to be found.
I searched for all activities between 7.9 and 8.1 miles in case the date/time was off (as I had that issue with a previous 310XT that didn't save date/time properly). No luck
I checked the Activities folder under AppData/Roaming/Garmin/Devices/#######.../ and see where my most recent .fit file from 11/3 had it's Date Modified time stamp changed to 11/4 and the time that today's activity transferred. The same is true for the .tcx file under History. the pending uploads folder is empty. Yesterday's activity uploaded to GC just fine and showed up.
Now I wish I hadn't set Ant Agent to delete activities after upload since this one is gone for good.
I did a subsequent 10 sec activity just now to see if it would transfer to GC and show up which it did.
Hopefully this was a fluke as I've never had an activity not appear but I wanted to put it out there in case anyone else ran into this. My theory is that it was a DST gremlin.