Advice for a spreadsheet macro that calls home?

A client wants to find out who is accessing some confidential data on his
machine.  Looks like an inside job, the IT staff reading an .xls.

We have a few approaches to this investigation (for instance, putting a
string token in the file, and using Snort to watch for it).

Putting a 'call-home' macro in the file seems like a good bet, since
the file could be pulled in many ways, but must be opened for
reading.  I'm thinking that when the file is opened, a network connection
to a server is opened, and then we know when and where it was opened from.

I haven't read any VB code since looking at the Laroux macro
virus.  But this seems like an easy bit of code to plant in an excel
spreadsheet.  Especially if i found some trojan/worm code to steal from.

Are there tools/worms that do this already I should look at?  Am I
over-looking some problems?

thanks,

marc bayerkohler
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