Symbols

Here is a preliminary list of the different symbols I have found:

Fog- seems to represent privacy and mystery
D-503 expresses that he likes the fog, but also is afriad of it

Hands- D-503's hands symbolize savagery and "primitive" life
D-503 hates his hands, but uses them to express his savage side

Birds- represent D-503's (and others') struggle to escape
D-503 makes numerous references to birds trying to break through the wall
(May also be an allusion to the classic "bird running into the window" joke)

X- seems to represent the unknown (x is a variable in mathematics)
D-503 keeps seeing this X factor in I-330

D-503 also mentions X as one of the axes of a coordinate grid (along with Y and Z), which may symbolize that he is trying to find his (metaphorical) location, but cannot because he only has one coordinate