Honestly, Marek hadn't mentioned Melinda in his note, but her response about them hadn't been contradicted. The witch didn't know very much about Melinda Bobbin: Marek's twin, recently upset over the betrayal by a friend (given the wanted posters either Terrence Higgs or Ella Svetkova), and as expected, she had threatened Felixa if she hurt Marek. There were the little stories Marek told, but overall Felixa simply wanted to help both of them. No one's mind deserved its privacy ripped away. And it came with the benefit of seeing more of Marek.
Even though Marek normally dressed fairly well and when not exhausted buttoned everything correctly and such, Felixa chose to dress casually, along with Pace, the knife Marek had made for her, though it was very well charmed to be hidden. With the choice before her - floo or apparition - the witch chose the latter for two reasons. First, in general, she preferred apparition to the floo, for the issue of ash. Further, it would take her immediately into, potentially, exactly where they were. Given their dynamic, that did not seem the wisest of choices, statistically speaking. Knocking gave them a small amount of time to instantly prepare for her, should they need it.
That took no time at all, and the witch smiled as she looked at the front door. She hadn't seen where Marek lived before. At that, Felixa knocked.