When the mistress asked how he knows that it wasn't her who saw them, I'm guessing he said it's because he put someone (the ML) to tail her, and we've seen him do that, but seems like he lied about her being at the gallery.
Also, now we know the ML knows her from the past, but the moles of the ML and the dead lover are different places, so my guess is that he's a sibling or some sort of relative.
When the mistress asked how he knows that it wasn't her who saw them, I'm guessing he said it's because he put someone (the ML) to tail her, and we've seen him do that, but seems like he lied about her being at the gallery.
Also, now we know the ML knows her from the past, but the moles of the ML and the dead lover are different places, so my guess is that he's a sibling or some sort of relative.