DaveO wrote...
....
The weird part is: if I give one of these VLOOKUPs a new
lookup_value, I get an answer, rather than a prompt for a
password or "access restricted" message. The substantially weird
part is: I can close the file and close Excel, unplug my network
connection, reload the file, use a new lookup_value and STILL get
an answer.
....

Excel caches data from external references in the file containing the
external references. This is just one of (way too) many things that
can make Excel workbooks huge.