Hi, There are many way to do this, it depends how subtle you want to be. In one example you could just say something like 'Guess I should get going now', this would be a subtle way to do it without making it obvious. If you want to embarrass them you could say 'I'll leave you two love birds alone'.

Does she know? . Must go. Things to do. Must go. See you later. I hear my phone ringing. ... I'll leave you two to it. I'll leave you two alone. .. I'm not comfortable being the third wheel.

Do you want to be blatant or subtle? Do you want them to be at ease, or make a little fun at their expense?