Error Search Code: Serial Number Not Found

I have the following code to search for serial numbers.

Private Sub CommandButton1_Click()
Dim Message, Title, Default, SearchString
Message = "Enter Serial Number" ' Set prompt.
Title = "Find Serial Number" ' Set title.
Default = "" ' Set default.
' Display message, title, and default value.
SearchString = InputBox(Message, Title, Default)

'SearchString = "Rob"
Set S = Sheets.Application
For Each S In Application.Sheets
With S.Range("A1:IV65536")
Set f = .Find(SearchString, MatchCase:=True, LookAt:=xlWhole, LookIn:=xlValues)
If Not f Is Nothing Then
f.Offset(, 3) = Date
Exit For
End If
End With
Next S

End Sub
I would like to amend this so that (a) if the serial number is not found I get a message box saying "Serial number not found" and (b) if the serial number is found, I would like it to highlight the relevant row (after inserting the date).