Comparing sheet values to external list, flag anything missing

You will need a way for Excel to get that list somehow. That could be copying that list to another sheet in the workbooks of interest, creating a defined Name that includes that list, hardcoding the list into a macro/VBA function, or possibly other ideas but somehow Excel needs access to that list. Using VBA you could have it specifically open that file but the problem is you need to open that VBA/Macro and therefore need it in each workbook or in the master workbook anyhow. If you have many different workbooks, to me it seems easy to open your 1 master worksheet and then do comparisons that way as opposed to copying it to every single other workbook. That said, if each 'new' workbook it generated based on a template, then just include that extra sheet or defined name in the template. Depending on the approach, the comparison can be easily done using built in functions and/or conditional formatting.