If you consider Aunt Cass brought Hiro food and that Hiro has a mini fridge and a bathroom in his room, the chances of Hiro leaving his room inbetween Tadashi’s death and Baymax activing become even smaller.

Especially if you consider Aunt Cass’ suprise when Hiro does come downstairs. He probably hasn’t been outside of his room at all during that time.

And also consider the fact that Hiro spent all that time in a room that only reminded him of Tadashi more, because it was Tadashi’s room too.