I have no doubt they had to piece it together. In my opinion, they pieced it together after the 911 calls were released on the previous Friday night, 3/16. Martin was trying to figure out Trayvon's password on 3/18, he was flipping through the phone numbers and noticed g/f's number, and called her.
1. LE would have taken the password off, or, he could have gotten it from T-Mobile - it was his account.
2. The timing of the call works out perfectly - 7:12 is the time TM started running - g/f calls him WHILE he's running and tell him to run? Convenient
3. Where is the call to or from Tracy and TM when TM was begging his dad to let him go to the store?
4. Where are all the call backs from g/f and the calls from Tracy after the 7:12 incoming call? The phone wasn't rendered inoperable because Tracy called the g/f from it.
5. Why didn't g/f come forward with this important information during the time the Martin's were begging for an arrest? Why didn't her mother make her? According to Crump, these two were very close, they were girlfriend/boyfriend and they were "dating". And yet, no one calls her before 3/18 - they obviously knew about her!