Katniss hears her before she sees her. She's always been light on her feet, has always had a hunter's tread and Johanna is someone she can hear easily. Is this a trick?

There's nothing to indicate the hotel is anything but a hotel but Katniss doesn't trust anything easy. If this is a Capitol trick, they're not going to lull her into security and make her feel safe just to catch her by surprise.

"Johanna? Is that you? Is anyone with you?"