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Could it be from the top floor of the garage at that low-rise complex on the east side of Mopac that's usually visible in the shots taken from Barton Creek Square Mall?
Here's a photo I took from Westgate Tower a couple of years ago.
That floodlit white building behind the Zilker Tree is the one I'm talking about.
If not from there, look at the below photo from Westgate Tower taken in the daytime.
To the right of the white building with the parking garage is a low-rise reddish office complex. Maybe the photo you're asking about was taken from there?
(Note: in the second photo you can see the pole that makes up the spine of the Zilker. The top of the pole is just underneath and between that white complex and the reddish one.)