I don't know the answer, and can only add that I was disembarked from 573 quite recently (a JFK-LHR service). But to help people a little here is an extract from the relevant map:



To help focus on this, T5C is the long purple blocks below B, the break indicates a ground level opening which isn't apparent inside T5C. The Qantas A380 often nest in 596.

Speculation: to prevent fire engines from being inhibited by passenger movements around 581?