You never have to remind your opponent about "may" effects on their own models. If they choose not to deploy their models with Ambush or use Counter...
It is the same justification that allows you to check to see if a unit is completely within 4" of a flag to score a flag at the end of a turn. If you...
When it comes to the end of the turn, if you think your opponent's warcaster/warlock are completely within 14" of a board edge, you check. It doesn't...