The appearance of Sketch was already hinted in the second-season trail, which contains a brief shadowy scene of a person looking at Maxxie through a window; the word 'sketch' is scratched on the wall next to the window.
The photo that Sketch/Lucy looks at whilst having sex with Anwar is the same photo that Maxxie has a drawing of shown in S1: E6 'Maxxie and Anwar'.