Mine and a friends 800's have done a similar thing in the last week (though ours had an activity in history just nothing in it) it must be some sort of bug due to an update it the only conclusion I can come too. Neither of us have had this kind of issue before.
So, forerunner 210 - I have today's MTB lolz on it, and I save the activity using the lap/reset.
I then look at the history and there's no record of the ride.
I then plug in to a mac, and there's no FIT file and the error log has been touched. I open the error log and there's no entry for today.
I unmount the garmin and fire it up, go to history, and low and behold, the activity has magically appeared.
I plug the garmin back in to the Mac and there is a new fit file with today's date. And it uploads to Connect just fine.
What could cause this? Garmin fluffs out writing the FIT file, but the Mac somehow automagically fixes it?