Q:

No environment texture but there is one in the reflections

...something strange here. I have a scene with a node setup with no environment texture, but in all reflections i see one. The world tab and shader nodes are empty and it should be a dark, grey background. Background under the render -> film section is also not set to transparent. In both evee and cycles. I want to find the texture, since i somehow lost it in a later version of my scene.
...i thought at first i broke blender, since all reflections were gone as i used the undo function. But somehow this secret environment texture, which is not shown anywhere else as in the reflections of the object in the scene.
Any idea why this is happening or/and how i get the exact image background back?

A:

You can see the built in HDRI by going to Material Preview, and turning up "World Opacity" under viewport shading options.

If you want to use one of the "preview" HDRI's that came with blender as a render-able HDRI, you can find them (.exr files) in the blender folder at ../2.82/datafiles/studiolights/world. The default (likely the one you saw) is forest.exr. Connect it to your world shader as an environment texture.
If you want other options, there are many free places to obtain them online. One of the most popular is HDRI Haven (https://hdrihaven.com/hdris/).

