While you are inside that highlighted area, you will encounter more Traces from that specific family.
Excluded Families: Oddities and Brilliant Foundables
There are no Landmarks for Oddities nor Brilliant Foundables. If you are looking for these specific Traces do not put up Dark Detectors within a Landmark as you will not get the results you are looking for. Instead, use Dark Detectors in an Inn that is not inside any Landmarks.
Where to Find Landmarks
The best way to find Landmarks of specific Exploration Families is to explore your local environment. Talk to other players and share information on where Landmarks for each of the Families are located.
If a Train Station has 13 or more platforms, the 13th Platform will be a Dark Arts Landmark.
Most (not all, but most) Dog Off-Leash Parks will be a Landmark for Care of Magical Creatures.
Suburban Sporting Grounds and Playing Fields often Landmarks for Magical Games and Sports.
Contrary to what you might expect, Zoos are rarely Landmarks for Magizoology Foundables.