Remember two things, I took that photo with a simple p&s camera with a small ccd sensor, it was not the best to take that pic but most of all I can guarantee you there is no small light dome in that direction ! Also the camera could not, save for a small glimpse, pick up the MW that extends through Sagittarius as you know. The MW would be glaringly bright, running almost overhead but not for that camera. That is why I am asking, ‘ what is it ‘ ? If its the sun it is quite a bit to the SW from where it sets ? It is interesting, thats why I showed that photo. But you are correct it sure is not like the photos that I googled which are probably the best of the best, with the best AP geared cameras. I wish I would have had my dslr handy !
Actually, I see quite a lot of the Milky Way in your photo. An award-winning picture it's not, but M8, M24, and the Scutum Star Cloud all show up just fine. The brightness of the light in question is part of what makes me doubt that it's the zodiacal light. Yes, the zodiacal light can indeed outshine the Sagittarius Milky Way, as I was startled to discover the first time I experienced it. But not by that big a margin.
The Sun, as you know, moves to the west as it sets, and this does not stop after the Sun goes below the horizon. Around the solstices it hits the horizon at quite a severe angle, so yes it would have moved quite a bit farther west than it was when it set.
The photo has to have been taken well over an hour after sunset. That early in the evening the whole sky would be lit up. I guess the real test of whether you're looking at the residual glow of the Sun below the horizon is how it evolved over time. If it was indeed twilight glow, it would have continued to change in direction, and also gotten dimmer fairly rapidly. That's true of the zodiacal light as well, to a lesser extent. An artificial source, by contrast, would normally stay in exactly the same spot and typically dim slowly if at all.
You may be underestimating just how far artificial light pollution can travel. Especially when there are clouds hanging over the light source, the glow of a city can appear quite bright well over 100 miles (160 km) away. The presence or absence of clouds changes the brightness greatly, which is perhaps why you only see this on some nights. I do see some prominent clouds at the right-hand edge of your photo. Lit up by something, presumably artificial.