Hi,
I've just read your post and a couple of things jumped out at me.
Firstly,
You say you drink the same amount every day. That should mean that (if your body is functioning in a steady way) you pee the same amount every day, yes?
So, does that happen?
On a day when you pee all night, have you been peeing normally throughout the preceding day? (Do you end up dehydrated)
Or did your body not pee during that day, and save it all to pee during the night? (Do you end up normally hydrated after peeing out a stored up excess from water retention)
I am speculating the your answers to those queries would give you a good indication as to what may be happening.
Additionally, just because you are eating low carb, doesn’t necessarily mean that your foods suit you. There are quite a few foods that affect thirst, water retention and kidney function. Salt is one, of course. Others include coffee and artificial sweeteners. There are lots more.
I imagine that different people have different reactions to different foods. In my own case, my body loves to retain water in hot weather, after eating carbs, and after certain medications. I usually end up adding electrolytes and salt because I get cramps otherwise, so it is v hard to guess whether salt causes water retention, since I would rather have water retention than cramps