Thanks for responding!
1) Are they sniffing and then raising their leg when the pee on the curtain? This is more in tune with marking.
I have seen one of them do this once, however it always happens at night when everyone is asleep
2) Do they pee in just small amounts, like a little trickle? or do they leave an actual puddle? The trickle is more in tune with marking.
Well the curtain is wet as is the door mat (they are mats that pick up wet dirty paws so are really absorbent) so I would think it is an actual wee, although their wees outside often look larger, so I am unsure, it is however a significant amount, if they mark at the dog park it is certainly less than what they are leaving inside.
3) Do you have any wild animals frequenting the yard? Some dogs urine mark when they smell animals.
We have had a fox go for the chooks, but the boys they chased it away and we get rats in the garden, which they also hunt, other than that nothing except birds
4) Are the curtains new? Some dog mark on new items. https://www.braintraining4dogs.com/members/archive/why-does-my-dog-pee-on-anything-new/
No, we moved to this place when they were 7 months old
5) Are they in any way reluctant to go outside in the evening? Have you noticed any hesitancy in going through the door specifically at night? Is it very dark once they are outside?
They have a solar light that lights up outside where they go out, they have no problems at all hooning in the dark, we have played outside in the dark with them from when they were really little so it wouldn't ever be an issue.
I would be monitoring them closely, closing the room on the way to the doggy door so you have only one room to focus on and making sure you always have a clear, unobstructed view of them at all times. Removing the curtain or elevating it in a safe way (with no cords dangling) can be a good idea for the time being, considering that it's hard to tell if they pee since curtains are absorbent. This will also prevent you from repeatedly washing them with an enzyme-based cleaner.
Unfortunately we have no internal doors except bedrooms and wet areas, so nothing to shut on the way to the doggie door, as I have mentioned, no accidents when we are all up, only happens when everyone is asleep, they have no problems taking themselves outside at night to poo and to do other wees, it is only ever the one wee inside when it happens. Luckily these curtains are not thick and I can just throw them through the wash.