Question What does URP Post Processing do with no volumes?

I'd expect it to do nothing, but that's not the case. In the image I've attached, the left is without post processing, and the right is with it turned on (toggling via the checkmark on the camera's Rendering section). The whole scene gets a little brighter, and the lines get a little crisper. This scene has no lights, the image is set up with it's own emission. The background is black (no skybox), no environment lighting. No Anti-aliasing on the camera or on the mesh

I've got a global volume and if I put PP overrides in there, they correctly toggle on and off as expected. But there's SOMETHING going on even with all the overrides off (or even the whole volume turned off). I can't seem to find any documentation on what changes Post Processing is doing by default. Some kind of Auto Exposure? Some kind of sharpening?

When it's turned on there's a clear shift as you hit Play that isn't there when it's turned off.

Thanks,

Rick