Hello,
I know about the feature where material auto-delete itself with 0 users. But I got materials without any user (I checked manually all objects) and they do not have fake user, but still they have some invisible fake user or something.
How to fix it?
These materials are created using hair tool.
Go to orphan data and purge the scene file. Normally though, closing and re-opening the file should clear out any unused materials.
“how to find all users of the material” I would like to know this too.
Activate the Amaranth Toolset add-on. There is a tool to search where a material or an image is being used.
I tried this, but it seems like materials have a user that doesn’t exist.
Could you share an example file?
Do you have linked files in the scene? Or appended things?
I had this a couple of times, and it ended up being old links that ‘stuck’ in the scene after making everything local.
maybe it helps,
rob
edit: Be sure to activate the ‘Material Utilities’ add-on in Blender.
It does what you want for the shared materials search, but it also makes assigning shaders a smoother workflow. Especially for ex-Maya users it looks familiar at least 
Try looking in the blender file option then, you may be able to “unlink” from there.