Oh dear. The game is a bit wobbly when events happen at the same time, so perhaps you unfortunately pressed the long rest button exactly as someone spotted you and combat started. I had something that sounds a bit similar once when my characters got stuck long-resting on the main map in the Underdark when I happened to start a rest at the same time as a cutscene triggered. I ended up having to reload a save from before the issue happened. I hope you have a save from not too long ago.

"You may call it 'nonsense' if you like, but I've heard nonsense, compared with which that would be as sensible as a dictionary!"