Harry put an arm around her shoulders and sat up to hold her more closely. "The wards haven't been triggered yet, but that doesn't mean they won't be."
What he didn't want to admit, to her or himself, is that Grindelwald was powerful in his time. The tracking spells he left at Ron's flat were developed after Grindelwald's time, but it didn't mean he wouldn't find a way to dispel them without Harry or Hermione ever knowing.