I'm using TexStudio 2.10.8. When I search for a string in "All Docs" it sometimes happens that the search text is found in a file which has been previously opened, e.g., in another project, but was closed at the time when the search began. The file is then reopened and appears both in the file tree to the left and with a tab above the text area.
/Klas
all documents which are opened once, are not closed internally until no reference on them is present (via include/input)
You can see which files are still open with view/list of hidden documents