Selina likes a fancy party. What can she say? Maybe it's the atmosphere (everybody's got their guards down). Maybe it's getting to rub shoulders with some of Gotham's elite (and scope them out a bit). Maybe it's just getting to step into some of the most lavish buildings in the city legitimately (and scope them out too).
Look she's not denying that she's used a gala invitation as an opportunity to plan a heist before. More than once. What of it? She likes what she does and she's good at it.
But more than that, she actually does enjoy just... being at a gala. She can admit that there's something very pleasant about dressing up nicely, looking her best, exchanging gossip with people who might normally look down their nose at her but tonight believe - at least for the moment - that she is worthy of their polite attention. She's aware she's not quite 'on their level', and in truth she doesn't want to be, but look she enjoys looking pretty and feeling fancy and putting on the little 'high society' act now and again. Call her vain, she'll take it. But there's no denying she's rather good at this too.
So tonight, she's quite enjoying herself. Sipping champagne, trading gossip tidbits, admiring the décor and a few treasures on display. It's been a pleasant enough evening, all things considered, and unlike some names she could name, 'stepping out on the balcony for a bit of air' is not, for her, an attempt to flee the party. It's really just more of an excuse to enjoy the view. Sometimes, Gotham's almost a pretty city, when you look at it from the right places.
Of course, she knows that doesn't hold up on a closer inspection, but that doesn't mean she can't appreciate the sight for what it is.
She's been standing there for a few moments, leaning on the railing and sipping at her champagne, when she hears the balcony door click open behind her, and glances over her shoulder to see who has chanced to join her outside.