(photo: credits)

The L.A. Noire Interactive Map is split between “official markers” and “unofficial markers“. According to the IGN website the official markers include things like landmarks and hidden vehicles, which you need to collect to achieve full 100% game completion, while unofficial markers are for other interesting locations like buildings with interiors, or other interesting landmarks that are not a part of the official list. Visit the IGN L.A. Noire wiki section for more details.