FM-15: strange item in Windows 10 taskbar

From time to time I realise a strange item in the taskbar:

The prsence of this item "Insets" does not depend ...

• wheter I have to many document tabs (resulting in a special drop down list far right in the tab-bar under » )
• whether the Welcome screen is selected or present in the tab bar
• a book is open and some document are open
• I run an ExtendScript in ESTK

I  have however noticed that it disappears when closing a certain document from the manay (about 12) open document. But when reaopening the book with all of these documents, the strange item is not there...

Any clues what this could be? I will also check Task Manger next time I see this strnage 'task'.