You shouldn't call a function if it shouldn't be applied.
You shouldn't test that after calling the function, but before

Sub M_show()
  if left("text"="\\?\" then y = F_RemovePrefix('text")
End Sub

Function F_RemovePrefix(s As String) As String
  F_RemovePrefix = mid(s,5)
End Function
I'd prefer, instead of these lines of code:

if left(s,4)="\\?\" then s = mid(s,5)