When Melter first visited, he probably only had a priliminary report on the group. Age, sex, name, knowledge of any capability including sorcery. Maytag's assessment for Dark Cell, most likely, wasn't finished and the main thing Melter knew when getting there was that a young, level 3 sorceress was available. I'd say they probably don't get too many candidates who are both young females AND level 3 sorcerers. There's also the matter that Suspira is a Genius and I'd easily presume that the Thin Man and Melter keep a track of those out there, particularly a level 3 one which is apparently quite rare.
Its possible that Danzig had managed to send in an updated report on the group, particularly on Maytag and her resistance to the bath spell, together with her successful assessment for Dark Cell, before he was overthrown and arrested.
However, I'm thinking that the Thin Man may have recognised Maytag's name when Melter told him, and its possible that The Thin Man hadn't read the report yet, waiting for Melter to filter candicates and present them to him after full processing. By the end of this book the Thin Man should almost certainly be revealed, probably with Maytag in his clutches and a ready introduction for Dark Cell.
Hopefully we get a big-ass sorcery battle somewhere in there.