There should be two reasons why this could happen:
- The process has exited unexpectedly early, such as it has processed all the files before the controlling thread begins to wait on it. This may happen, for example, if all the files scheduled for the worker are .txt files, which it would skip.
- The process has exited unexpectedly, perhaps because it crashed, or for some other reason, such as it was unable to open the shared memory etc.
From your output, where this occurs to two of the 4 workers, then it's either case #1, or there are two files of death out of the 541 that crashed worker process that tried to index them. Perhaps the Windows event log captured the crashes, if indeed that's what occurred.