Unless that wasn't Pandora, which is also possible.
Assuming I got everything right, Pandora was at Heka's (hearing about the lie she'd passed on to herself) on Monday or Tuesday, whereas the Jerry encounter (where she might've heard about Susan and Diane) was on Wednesday night. This encounter between Sarah and Box apparently takes place on Thursday, so she probably wouldn't have had too much time to do more research, much less about Adrian's possible descendants, since that would probably involve talking to him. Of course, if she knew about his sex life and the unexpected appearance of children before, it's quite possible that she only now, after hearing the truth from Heka, put two and two together.
Another possible interpretation of "of course they were his" could be that "they" are more powerful magic users in general and "he" is the guy with the mantle of Heka. That was my first idea. Though that probably wouldn't quite explain why this problem affects Adrian as much.
I kind of can't shake the feeling that Dan has a bigger plot twist planned. All the speculation in this thread seems too straightforward – Adrian leading a sex life, accidentally fathering children even he doesn't know about (or doesn't want to believe they're his), based on how vehemently he denies his fertility, and them being ancestors to Susan, Diane and potentially others. It seems too normal. Quite frankly, I'd be a bit disappointed if there isn't more to this.