This linking is a (useful) feature (by dropping to a box automatically) with the aim to avoid redundant objects. This means you could one single objects use on several places/sheets - all properties unless size/position/... are the same.
You could these link create per shift + ctrl + drag or per menu as insert as link. Also you could delete these links - you will ask if you want delete this link or the object or per menu-point linked objects.
You could search linked objects in document properties / sheets and click on Object ID (is by linked objects always the same).
The problem is that the Document Properties|Sheets only shows objects directly contained by the sheets. If an object is only hosted in a container, then it does not appear in Sheets. You have to open each container in turn (which you can do from the Sheets tab) and check the object IDs.
the last part of your post is very interesting - I have been searching for some way of finding out which objects are linked - but it doesn't seem to work, or I'm doing something wrong:
- I open the document_properties
- I look at the tab "sheets" where the lower box displays the objects
<=> Every object_ID seems to be there only once.
Can you please explain that?
Thanks a lot!