[Baymax had gone out of the house in a hurry, so he hadn't taken note of Aunt Cass' location.

Going in through the garage is probably a safe bet, she's likely gotten used to all sorts of strange noises coming from it while Hiro works on one project or another.

He nods and gently scoops Hiro up. This is one of the things he's been programmed for, if a patient is unable to move, then he's designed to be able to safely carry them where they need to go.

Baymax makes sure to hold Hiro at what he believes is a comfortable angle and his air pressure adjusts so that his patient isn't sinking into his skeleton. It probably feels something like lying on a giant balloon or pool raft.

Once he thinks that they're ready to go, he takes off towards the Lucky Cat Cafe, going as fast as his stubby, round legs can take them.]