I use to think that it was because the T-800 ends up calling her, but that was after Reese and Sarah go back to the hotel. The detective mentions that she was waiting for her mom to come from Big Bear. There was nothing that the audience knew that would suggest that the T-800 knew they were at the police station.
In Terminator 2 Judgement Day, this was always covered as there was no time when we did not know the T-1000 knew where Sarah and John were.
According to the film's official novelisation, after it had repaired its eye, it simply phoned all of the Los Angeles police stations until it found the one that had Sarah. I think we can assume that it used a simple ruse such as pretending to be her mother or another policeman.
Starting with the As, Terminator rapidly dialed every police station in Los Angeles until it reached Rampart Division.
For the record, the mention of her mother in Big Bear was almost certainly intended as a Chekov's gun to explain to the audience both Sarah's and the Terminator's next moves; traveling up to stay in a motel near to her mother's house and the Terminator's murder and replacement of her mother.
