Update: ...Alright. So I let her out of the room, and she had fun exploring the first floor. When I was just about to pick her back up and put her back in her saferoom, she jumped away and ran up the stairs.
Now, this kind of came as a surprise to me... my basement stairs have no doors, but I kind of figured, 'hey, each of these stairs are taller than her- no way she'd be brave enough to scale all of them at once'. Except, well, she did.
;;; And when she got on the second floor, she kind of behaved weirdly... she would compress herself against the floor and slink away whenever I pet her, and when i'd walk out from a door she'd act really spooked. When I called her name, she didn't reply either (usually when I call 'Mika!' she'd mew back, even when she was hiding). I didn't want to pick her up because I knew that would scare her even more and she'd probably escape anyways, so in the end, I finally lured her back to her room with her toy.
I guess Mika's not exactly ready to go up to the second floor yet... I'll probably let her explore the first floor again tomorrow, but I'll block the basement stairs when I let her do that. I'll probably try the second floor with her in a few days, but for now, i'm keeping it off limits.