image review – Can anyone please tell me what is this small line in my picture?

I was taking a photo with the nikon D5600 , of 2 people sitting on a bench. As you can see, there is the Moon in the background and a few stars. But what is the small line near the top ? I hope i am not seeing UFOs.
The photo details are
Exposure = 2.5 sec at F/5.0
Focal length = 18 mm
ISO 8000

enter image description here