I just noticed that I have certain files wich seem to be in the database but don’t have any Group information. So I can’t find them anywhere in my hierarchy but I can find them with search.
Added a screenshot of the search result for such a file.
Is there any place I can view all those “lost” files?
How can I prevent this happening?
Database Properties are: “Exclude Groups from Tagging” = false, Share = false, Create Spotloight index= true
Looks like the files may be in the root (top) level of the database. You can confirm if this is the case in th Three Payne view.
Greg is correct. These are in the root of the database.
If you select Go > Top Group from the commands menu you’ll be taken to the root of the database.
Or, press the little house icon (middle icon in this image) on the toolbar, if you have configured the toolbar to show it:
Drag the files out of the root and into another group.
It’s easy to drop a file into the root if you drag it to the database and drop it somewhere in the margin of a view (e.g., the group pane in Three Pane view). It’s a good idea to periodically look in the root for these little droppings 
Indeed I see the files in the root, didn’t know this existed so far.
Thanks for all the helpful answers.