The traditionally shy creature was spotted inside the Hauxley Wildlife Discovery Centre run by Northumberland Wildlife Trust.
Receptionist Caz McMahon found it sitting on the bench inside, calmly admiring the view of the lake through the window.
She opened the door to try and let it out, but, unperturbed by her presence, it continued to enjoy a rest before deciding to take a look further inside by climbing up the corridor’s gabion wall and peering through the glass door of the Lookout Café.
Caz said: “Hauxley Wildlife Discovery Centre is a great place to work anyway, but I was so delighted to see the squirrel just sitting on the bench.
"Then, when he climbed along the gabion wall and peered through the door, I just held my breath with excitement.
"It’s just one of those things you don’t see every day, but here’s hoping he comes in again.”