Peter uses Brian's car and says it's his own fault for keeping the keys in the visor, but because of the way the Prius ignition system works, requiring only the presence of the key rather than it actually being placed in the ignition means it's basically ready to run whenever anyone gets into it. Hiding the key is irrelevant.
When Chris, Stewie and Brian enter Taylor Swift's mansion, Stewie says "Your house is way nicer than Anne Murray's. We broke in there too." But Stewie and Brian (who he was with) never broke into her house.