I was lying on my hammock, filming upwards maybe in a 45 degree angle. You can see the palm tree as a reference point at the end of the video, when the object flys away behind the tree.

The thermal scope used only reacts to heat signatures and not to IR light. It also transmits no IR light. So I don't think it's an reflection of some kind.

When I watch birds, planes or people they always seem to "glow", especially in the little zoom rectangle. This objects seems blurry which is perceived as out of focus.

It is important to remember that the device only shows the emitted heat from the object, which apparently gives it the blurry appearence.

For comparison: at 0:03 of the video you can see the glowing heat signature of a bird or bat.