It's easier to get outside the fence than back in. So, after picking up the Silver Dragon, save your game. Then try this trick, which is also shown in a short video. (If the video doesn't open automatically, right-click the link and choose 'Save Target As...' to download it.)
Stand facing the tall, red metal tower just outside the fence. (This should actually work on any of the fence panels, but it seems to be easiest to set up here, where you have the red tower as a landmark.)
Step forward so Lara is touching the fence about halfway between the red tower and the right side of this section of fence, as shown above. Walk forward into the fence, jiggling left and right just a bit as you move forward. When you hit the fence at just the right angle. . .
. . . Lara pops through to the other side.
There isn't much to see out here, but you can move around the square walkway and admire the sunrise.
When you're ready to come back in, move to that same weak spot in the fence and repeat the forward jiggle.
It takes a bit more finesse to pop Lara back inside, but the maneuver is basically the same. If it doesn't work, just reload your last save.
Thanks to rr_carroll of Tomb Raider Tourist for sharing this discovery.
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