Macro that Matches Scanned Data

Possibly...

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim lRow As Long, rg As Range, c As Range
    Dim b As Boolean, s As String

    If Target.Column > 1 Then Exit Sub
    
    b = False
    lRow = Sheet1.Cells(Rows.Count, 3).End(xlUp).Row
    Set rg = Sheet1.Range("C6:C" & lRow)
    With rg
        Set c = .Find(Target.Value, Cells(6, 3))
        If Not c Is Nothing Then
            s = c.Address
            Do
                If c.Offset(, 3) = vbNullString Then
                    c.Offset(, 3) = Target.Value
                    c.Offset(, 4) = Now()
                    b = True
                Else
                    Set c = .FindNext(c)
                End If
            Loop Until c.Address = s
        Else
            b = True
            
        End If
        If b = False Then
            Sheet1.Cells(lRow + 1, 3) = Target.Value
            Sheet1.Cells(lRow + 1, 4) = Now()
        End If
    End With
End Sub

...placed in sheet2 code module.

Well I just realized I said row 3 and 6 instead of column 3 and 6. I'm using R1C1 because my employer requires it. Sorry for the confusion. This looks promising, but I'm not sure what I need to update to resolve my mistake.

See the attached file..

Oh nice, you saw through my initial mistake! This is great and is so close to what I need. It's filling in column 6 when it matches a pre-filled column 3, but it isn't doing the initial "scan in." For example, if sheet 1 is empty and I enter a 4 on sheet 2, I need it to fill out out column 3 on sheet 1 with that unmatched 4 on the next "empty" row. Does that make sense? This is extremely helpful, thank you for your help so far!

Try this...

Private Sub Worksheet_Change(ByVal Target As Range)
    Dim lRow As Long, rg As Range, c As Range
    Dim b As Boolean, s As String

    If Target.Column > 1 Then Exit Sub
    
    b = False
    lRow = Sheet1.Cells(Rows.Count, 3).End(xlUp).Row
    Set rg = Sheet1.Range("C6:C" & lRow)
    With rg
        Set c = .Find(Target.Value, Cells(6, 3))
        If Not c Is Nothing Then
            s = c.Address
            Do
                If c.Offset(, 3) = vbNullString Then
                    c.Offset(, 3) = Target.Value
                    c.Offset(, 4) = Now()
                    b = True
                Else
                    Set c = .FindNext(c)
                End If
            Loop Until c.Address = s
        End If
        If b = False Then
            Sheet1.Cells(lRow + 1, 3) = Target.Value
            Sheet1.Cells(lRow + 1, 4) = Now()
        End If
    End With
End Sub

That's absolutely perfect, thank you so much for your help!

You're welcome!