Investigation still progressing. Reports of this so far involve Virus checkers.
CANNOT reproduce it artificially by locking the file using Powershell:
(1) Run new job on a Windows slave.
(2) Open a Powershell Window and exclusively lock the file:
Set-ItemProperty $myFile -name IsReadOnly -value $false
$file = [System.IO.File]::Open($myFile, "Open", "Read", "None")
(3) Run the job again and see a different the error:
ERROR: P4: Task Exception: com.perforce.p4java.exception.P4JavaException: com.perforce.p4java.exception.P4JavaException: hudson.AbortException: P4JAVA: Error(s):
file 9.c operating system write error: C:\filestore\Jenkins\JOBS\workspace\MYJOB\MYFILE.txt (The process cannot access the file because it is being used by another process).
(4) Unlock the file