My guesses:
*Pre-arranged meeting - she saw this person at the gym, or ran into them at a grocery store/convenience store or gas station or even pulled up beside her in traffic
*Someone did call - but it is not raising any suspicions as there could be patterns of this person calling and showing on caller ID and call logs - maybe the call was made the day or evening before
*Someone put a note in her gym locker or on the windshield of her car or slipped under the door/taped to door of her house or note in her mailbox
*Someone, somehow got her attention at work
My bet is on a crime of passion - whether work place related (jealous of her successes and/or homophobic) or a former partner or someone new she blew off.