During Day 2, after Marie Warner had finished her business at R&J Lumber, she was told by Syed Ali to drive to the rendezvous point. On the way there she drove through Sylmar, and received a call from her sister Kate. Kate asked her what was happening, and Marie worked out that CTU had found her dead fiance and were tracing the call. She threw her phone out of the window where it was run over by another car.
The agent that was tracing the call told Jack Bauer they had narrowed her location down to a 10-block area of Sylmar. Jack then ordered him to tell CHP to close off the entire area in order to find Marie. ("7:00pm-8:00pm")
Background information and notes
In reality, Sylmar is the most northerly district of Los Angeles, in the San Fernando Valley. This would place Norton Airfield somewhere north of L.A.