Most interesting - I used Scanf.Scanning.from_file as the
documentation states that it uses block reads that are easier on the
I/O system than Scanning.from_channel. I rewrote using instead the
standard Pervasives.open_in and the associated input_line and
Scanf.sscanf; this seems to work fine for my purposes.

Olivier seems to have hit the nail on the head: if the end of a file
is not reached with Scanning.from_file, it stays open with essentially
no way to close it (other than artificially scanning to the end). I
think this issue is worth a bug report so I'll see if I can submit
one.

Thanks for helping to resolve this.
-Chris