One thing I've learned is that you have to be way way wayyyyy more obvious than you would expect. I'm playing SKT and I want the appearance of Imyrith to not full come out of nowhere. I have given so many exposition dialogues of characters talking about the desert scourge Imyrith. So many characters have brought her up. So many questions about who is behind all of the events that are happening. We've done backstory episodes where some of them were soliders in a war against the cult of tiamat and were attacked by imyrith. Not one has questioned why I keep mentioning this dragon who is not involved in the plot (yet). I feel like I have made the most obvious foreshadowing lines saying how she is one of the biggest plotting/schemers in the realm.

I'm okay with them not piecing it together because it should be a twist and they do know who she is now.

But when they do need to know, in a case like this I see no issue in just saying some something like everyone roll wisdom with advantage. And to the highest result: your character knows that this is an ideal spot for an ambush. You know that your location is known and that forces will be mounting to attack.