Worksheets Referenced in Formula, but not hidden

[bound4h]

I have a 2013-2019 marcro-enabled workbook. In it, I have formulas which reference a tab (worksheet) called "Lookup". However, when I rightclick, Unhide is greyed out as there are no other tabs to unhide. How can this be? The file is not password protected, nor is VBA password protected. I checked all of the VBA code and nothing refers to this tab. See below for an example of the reference:

=IFERROR(INDEX('Lookup'!$K:$K, MATCH(P147, 'Lookup'!$J:$J, 0)), "")

I don't understand how the file can actively refer to a tab that doesn't exist. And to be certain the link dynamic and not static, I can updated cell P147 and the formula calculates a new value and not an error.

 

[Z51]

The sheet was likely hidden using VBA, for example:

Sheets("Lookup").Visible = xlVeryHidden

To see it again, you have to unhide it using VBA, too.

Sheets("Lookup").Visible = True