Jealous fangirl is pretty funny too.
I dunno, he had it started figuring out once he was talking to her this chapter. Before that he didn't seem to know anything about her past her name and how strong she was, so it'd make sense that he didn't know she knew Rese. Doubly so since Rese almost only talks about what she wants to do with him and Seren was probably avoiding him cause angy.
See that's the part I don't get. To me that would be the exact kind of person that would get investigated, even more so as they are a Hero Candidate.
It also kinda ignores that Seren should be a known associate and sparing partner of Rese for better then a year. So even if you ignore Seren's solo activities, and rank as an adventurer, he would have know about her peripherally thrue Rese.
His entire character concept is about leveraging a web of interpersonal relationships to solve problems or gather teams to accomplish a goal. Net probably has access to informants that could reasonably collect the three sizes of any woman he wanted.... not that he would want to, but could if needed...
So I just find it unreasonable that he would have little to no intelligence on a person that is powerful enough to be ranked and also maintains a rivalry with with someone he thinks of as a friend or at least a close associate.