Although his question of 'who' remained unanswered, Zeus brought the car to a halt before Poseidon could jump out and hurt himself. He even stayed in the car as he was told because - and he wasn't ashamed or embarrassed by it - he kind of liked it when his brother was being a bossy bastard.
Unbuckling the seat belt, the Olympian opened the window on his side and lit a smoke, perfectly content with just watching the show. He could bully his brother into telling him what this was all about later.