If you have a very bright light behind you, it can enter the camera and mess the metering up. I think there the first lot of Canon 5DIIIs were there was also light leaking into the image during long exposures through the viewfinder.
I assume they have fixed that now.
Some top of the range cameras allow for the viewfinder to be closed via a small switch, and some cameras come with an attachment to slide over the viewfinder to block the light. Putting your hand over the viewfinder whilst measuring the scene will show if the ambient light behind the camera is affecting the metering.
I would never put the Long Exposure Noise Reduction on in camera for a couple of reasons, firstly it can slow you down, as on my cameras at least, the camera has to take a 'blank image' for the same time as the original long exposure you have just done to subtract any noise generated by the sensor during the long exposure. So on cold nights it doubles the time to take one picture. And it also runs the batteries down quicker because of that. Secondly, you are better applying any NR in an image by image basis during editing imho, rather than the one setting fits all that may be applied in camera. No need to apply NR if it doesn't need it, and if the camera gets the amount of NR wrong, applying more NR is not the best idea if you can just apply it correctly once.
An extra thing I have just remembered, if you are set up on a scene as the Sun is going down, use Auto WB, as it will change as the Ambient light changes, and take into account the artificial lights as they come on. Once all the light in the sky is gone though, I change to an appropriate WB for the artificial lights in the scene to be consistent. And there is normally ambient light in the sky a bit longer than you might think, as a long exposure can still record light in the sky our eyes can not see. Of course, if you shoot RAW you can change the WB during processing with no degradation to the image, but you should always be trying to get it as good as you can in camera.
A remote control is a very good investment, and you can normally get a basic one for a couple £s for most DSLRs. Some newer cameras can also be controlled by mobile phones/tablets over WiFi, but the camera will run down quicker because the WiFi is using a lot of power, and your phone/tablet is also running down its battery. Something more to worry about when it is very cold, as batteries lose their power so much quicker in the cold.
If a camera as Live View, then you can use it to focus more accurately, if there is enough light,
in any part of the scene, rather than just using a focus point, which can be a pita when your camera is on a tripod and the thing you want to focus on is not under a focus point.
It also enables you to easily manual focus when you need to, by using the LV magnified view to more easily check the focus. And indeed with the right camera you may be able to use the touch screen on your phone/tablet to touch to focus, and possibly change camera settings, but with the hit on batteries mentioned above.
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