If you want to see explore the depth image scans that led to this, there is an interactive viewer here. The alleged hidden door is on the north wall.

When this first went around a few months ago, I spent a few minutes poking at in Photoshop. I'm just some random dude with an image editor, but I found it pretty compelling. Here's the whole series and I'll walk through what I did.

North wall

This is the depth image of the entire north wall. I've highlighted where the alleged doorway appears.

Doorway

This is a zoomed in region just showing the doorway area. Note that if you look at the corresponding full color image, there is nothing visible that lines up with that frame.

High pass

I ran a high pass filter on the entire image. This "flattens" out larger but more spread out changes in depth so we can focus on the more fine-grained depth changes around the edge of the doorway.

Curves

Then I adjusted the curves to highlight extremes in depth. In the original image, farther areas are lighter and nearer are darker.. (Or maybe that's backwards. Either way.) Areas at middle distance a middle gray. After adjusting the curves, near and far areas are white, and the middle distance is black. Extremes in depth are brighter.

Levels

Then I adjusted the levels to bring up the contrast a bit.

Zoomed and enhanced

Here's the resulting image. Call me crazy, but that looks like a post and lintel doorframe to me.

I just want to stress that aside from the red rectangle in the first image, I made no local or selective changes to the image. This is just running very basic processing filters on the entire picture.