What did I just see ("line in the sky")

It's about 9:30 p.m. local, and dark. We just got back from the cottage.
Unpacking, I looked up and saw what I thought at first was a faint jet
contrail, light by the sun that had gone below the horizon.

The line ran, as far as I can judge, from due south towards north, and
stopped at 90 degrees overhead. There was a brighter part, about the length
of my outstretched thumbnail, just at the 90 degree point.

I went inside and got a pair of binoculars to have a good look, because the
bright part was not moving, relative to whatever quite bright start/planet
was close beside it. What I saw did not look like a contrail at all - it
was all the same width (very thin) and brightness, not spreading out at all.
The bright part was, well, brighter, but not much if at all thicker.

Then it was gone. The whole thing, all at once, gone. I saw it for about 5
minutes total, from unpacking the car to binocs.

Just went out to have another look - not there.

What did I see?