"The portal? Wait, do you mean...the last thing you remember is shortly after rebuilding you?" Hiro asked.

He put a hand on his chin, looking Baymax over thoughtfully. "Well, that's not unreasonable. Once you leave a dimension, you leave it's time relativity, so discrepancies could theoretically happen," he said. "...or alternate realities since...I don't know, would you have mentioned being brought to another dimension unprompted if you somehow got back?"

He knows Baymax wouldn't hide something, he's barely understood lying as a concept, but just not saying something? Yeah, definitely could have happened.