I found it. It's in "QV Server and Publisher" in chapter 8 (logs)
The actual file (it is named "In_use.txt") is located inside the QDS_logs, in the subfolder for the particular task for the particular date.
Long version: Inside "DistributionService/1/Log", there is a subfolder with a datestamp and inside that, there is one subfolder for every single task that was run on that day - and inside the subfolder for the qvw (inside every one during the time it's in use, I guess), there is that file.
P.S.: The trouble is, those subfolders are quite hard to search since the filename starts with a number and contains the complete filepath...
P.P.S.: Oh - and, by the word of the book, this file says only that the task is being executed - so it's not sure that is the file is not there, the qvw is not currently open and there won't be any confusion when I save the file and someone else is currently working on it...