Invisiable collections aka "groups" in 2.7. How can I make them appear in outliner?

Where they go after you Ctrl+G ?

Hmm never used ctrl-G… but this is weird… seema to be added but not anchored/rooted (?) in the standard Scene Collection ??. Try the Orphan Data view in outliner (broken heard)…the new colections its’ somewhere… is this a bug ? Becuase adding a collection in outliner does what i expect…

I found them all listed in Blender file part of outliner . I kind of accustomed to Ctrl+G because it’s what was making “groups” in 2.7

So maybe just a remnant of the old groups…

I once had them be adding as sub-collections for a last collection in the list. Now couldn’t figure out why they stopped to go there

No, this is by design.
It is quite useful to have collections which are not shown in the outliner. I understand it sort of like stashing collections. The asset browser might make this more accessible in the future but it is nice to have even now.

For example if you want to group all objects with a certain property without cluttering the outliner.
Lets say you want to be able to easily select all red objects. Put them all in an invisible collection called “red”.

Or for putting all objects you use in boolean operations into a collection.

Or perhaps you want to get rid of certain collection in your current scene but want to keep it because you plan to use it in a second scene later on. You simply RMB the collection in the outliner and select “unlink”. The collection is then gone from the scene but still in the file.
Later on you create your second scene and you can simply link the collection to you new scene.