Formulas not finding referenced data correctly.

I have built a "database" in Excel 2003 for a customer. The Workbook has many sheets. Most of the sheets are protected and hidden, and are only used to store the data. The user needs only a few sheets to enter data and one or two to retrieve the data in the format needed.
To accomplish this I have built a "summary sheet" where the user selects a given name and date from drop down lists to display the data for that particular person for the date range selected. I have used various VLOOKUP formulas in the necessary cells on the sheet and locked all the cells except the drop down lists for the name and date.
I tested this on my computer and everything worked well. After installing it on the customers computer, the customer noticed that not all the data he was entering was showing up on the summary sheet. I thought that the customer may have inadvertently deleted or modified the formulas in the "summary" sheet, but when we looked, the formulas were in tact and seemed correct. we noticed that although the formulas in the "summary" worksheet seemed to refer to the appropriate cells in the "data" sheet(s) the data in those cells was not showing in the summary sheet.
I had the customer copy the summary sheet and the sheet containing the "problem" data into a blank worksheet so I could look see if I could find the problem.
When I opened the Workbook, I was surprised to find that I was able to display data that was not on the sheet that he had pasted into the workbook that he sent me. All the formulas referred to the full database located on HIS COMPUTER as shown in the following example.

=VLOOKUP($B19,'C:\Documents and Settings\Owner\My Documents\[Database 2012.xls]Pay'!$A$4:$HI$2196,MATCH($D$8,'C:\Documents and Settings\Owner\My Documents\[Database 2012.xls]Pay'!$3:$3,0),0)

I have no such document on my computer in that location.
How is excel retrieving this data from worksheets that are not even located on my computer. (I do have a version of the same database on my computer but it is on a completely separate drive.)
I suspect that the formulas in the customer's workbook may not be getting the data from the place we think they are.
Any help would be greatly appreciated.

I would go back to the file on your machine and see what you have in that same cell. My guess you have linked it albeit inadvertently.

I had "my copy" up on my screen and was talking to the customer on the phone while he had his open. we looked at the cells in question and the formulas were identical, that is, they each referred to the same sheet in their respective workbooks without referring to any workbook by name (the worksheets in question are named "Summary" and "Deductions"

=VLOOKUP($F$2,'Deductions'!$A$4:$DE$147,MATCH('Summary'!H24,'Deductions'!$A$2:$DE$2,0),0)

notice that there are no [ ]s in the formulas. How could I have "inadvertently" linked his copy to mine without the reference showing in the formulas. Also, even if I HAD linked to another workbook an a remote computer, how would excel retrieve the data? would I not get some sort of error message stating that the source was not available??

More info:
As mentioned, the VLOOKUP formula(s) in question on the "Summary" sheet refer to cells on the "Data" sheet. When a value is entered in the cells of the "data" sheet the value does not appear in the cells of the "Summary" sheet. Could this have something to do with any of the following settings:
Automatic vs Manual Calculation
Update Remote References
Save External link Values