Answer:
Percy got away from Grover when Grover had to go to the bathroom.
Step-by-step explanation:
In the book it says,
"Confession time: I ditched Grover as soon as we got to the bus terminal.
I know, I know. It was rude. But Grover was freaking me out, looking at me like I was a dead man, muttering "Why does this always happen?" and "Why does it always have to he sixth grade?"
Whenever he got upset, Grover's bladder acted up, so I wasn't surprised when, as soon as we got off the bus, he made me promise to wait for him, then made a beeline for the restroom. Instead of waiting, I got my suitcase, slipped outside, and caught the first taxi uptown."
Hope this helped!
Stay safe! <3