I've owned my European rat snake for a couple of years now.
I keep her at 28-29 hot side down to 22 cool during 8 months of the year but I brumate over winter.
I would suggest that given the time of year that may be the current issue. He/she is looking to sleep through the winter.

In my experience this species is really timid. I basically added more and more hides and cover to the setup and now see her out and about most days. But even with plenty of hides when I first got her it could be easily more than a week between appearances.

When I first fed her she initially was a reluctant feeder, and when she started feeding would grab the mouse and drag it back into a hide to eat out of sight. She was really nervous and on one occasion when I moved around the room too much she dropped it and wouldn't eat for a week.

I would suggest if you aren't able to provide the 5-10 degrees for brumation you add as much clutter to your setup as possible, top up the water and leave him/her alone for at least a week.
After that take your mouse and leave it somewhere in the box overnight for you snake.

I'm sure you know, but this is not a species that thrives with constant attention.

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Her water bowl is even hidden behind a group of plants and covered in a couple of pieces of slate so she will use it regularly. . The more clutter the better for this species.

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Sorry for the spam but another thought occurred to me. Weekly weighing is probably excessive at this point and the stress will make it less likely he will eat.

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26 is not warm enough.
More importantly, this is a temperate species that wants to brumate. Provide the proper care it needs by brumating, and in the spring it will feed like a machine.

 

Forgot I never replied to this.

Yep, brumation was the solution. 5 weeks brumating between 10-15 degrees Celsius, and keeping him in a small container that was just about his length, with blacked out plastic because he's shy.

He's one of my best eaters now!

 

Brumation is almost always the answer with Zamenis.