A VBA "solution": make no claim on its robustness but works for sample provided,
See sheet2 of attached
Sub Find_Missing_InMem()
Dim ws As Worksheet
Dim inArr() As Variant
Dim lArr() As String
Dim inRng As Range
Dim l(5) As Integer
lr = Cells(Rows.Count, "A").End(xlUp).Row
ReDim inArr(1 To lr, 1 To 10)
Set inRng = Range("a2:A" & lr)
Range("c2:c20000").ClearContents
n = 0
For Each cell In inRng
n = n + 1
If InStr(1, cell, ".") = 0 Then
inArr(n, 1) = cell + 0
Else
lArr = Split(cell, ".")
For j = 0 To UBound(lArr, 1)
inArr(n, j + 1) = lArr(j) + 0
Next j
End If
Next cell
lastmax = 1
lmax = 1
For i = 2 To UBound(inArr, 1)
For j = 1 To 4
If inArr(i, j + 1) = "" Then Exit For
Next j
lmax = j
If lmax <= lastmax Then
If inArr(i, lmax) <> inArr(i - 1, lmax) + 1 Then
If Cells(i + 1, 1) <> "" Then Cells(i + 1, "C") = "Missing"
End If
End If
lastmax = lmax
Next i
End Sub
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