Inscribing Cells: Run A Macro On Enter Keypress, That Would Execute Different Code Depending On That "inscription" That Would Be Invisible To User

Is there any way to "Inscribe" a cell? I would like to run a macro on Enter keypress, that would execute different code depending on that "inscription" that would be invisible to user. I could use some properties of . Validation property like this:

Private Sub EnterPressed
'following code to ensure proper functioning of Enter in any other Worksheet
If ActiveSheet <> mySheet 'MySheet is global Variable then
ActiveCell.Offset(1,0).Select
exit Sub
End If
'now the real code
If ActiveCell.Validation.InputMessage = "1" Then
ActiveCell.Offset(0,1).Select
Else
'something else
End If
End Sub

The problem is, I use Data Validation and Conditional Formatting, so can't use any of these properties.