Hi nilem, works great! Thank you!!

Here's the final code (w/minor aesthetic additions):

Option Explicit

Sub Update_Master()

Application.EnableEvents = False
Application.ScreenUpdating = False
Application.Calculation = xlCalculationManual

Range("Master!A:A").ClearContents

With Range("Master!A1")
    .Formula = "MDET"
    .Font.Bold = True
End With

Dim x, rez(), wsh As Worksheet, i As Long, k As Long
Dim mdet As Range
ReDim rez(1 To 100000, 1 To 1)

On Error Resume Next
With New Collection
    For Each wsh In ThisWorkbook.Worksheets
        If Not wsh Is ActiveSheet Then
            Set mdet = wsh.Rows(1).Find("MDET", LookAt:=xlWhole)
            If Not mdet Is Nothing Then
                x = wsh.Range(mdet.Offset(1), wsh.Cells(Rows.Count, mdet.Column).End(xlUp)).Value
                For i = 1 To UBound(x)
                    If IsEmpty(.Item(x(i, 1))) Then k = k + 1: rez(k, 1) = x(i, 1): .Add k, x(i, 1)
Next i: End If: End If: Next wsh: End With

If k = 0 Then Exit Sub
With [a2].Resize(k)
    .Value = rez()
    .Sort Key1:=.Cells(1, 1), Order1:=xlAscending, Order3:=xlAscending, Header:=xlNo, MatchCase:=False
End With

'Additional code

Cells.EntireColumn.AutoFit

Application.EnableEvents = True
Application.ScreenUpdating = True
Application.Calculation = xlCalculationAutomatic

End Sub