She isn't scared, she is flirting with someone. Hiding in the elevator to prank him, jumping out, waving arms, that doesnt look scared at all, she's fooling around with a guy. A guy that probably has the keys to the hotel doors (manager, managers son, someone who stole the managers keys for fun, etc).
They then went up, had the shitty idea of taking a bath in the water tank, she died by accident, he got scared, removed her clothes and everything else in panic.
Solved.