You're right.
The secret basement lair is locked down even at the best of times. With the power cut, KAI is probably offline, but that would mean access probably is as well, so unless the intruder can somehow force the doors and just happens to have rappelling gear capable of 200 or so feet of descent (minimum), they likely won't be going down there.
I would assume there's some sort of emergency access / exit in case of a catastrophe in the lab complex, but it's probably concealed top-side, otherwise there's absolutely no point in a secure access elevator, so Artemis could get out and up, but the intruder would be unlikely to be able to make their way down.
That means assassin-bot stalking is still on the table.