Talking about symbolism in The Curious Incident is a little tricky. Christopher goes out of his way to intentionally include some imagery in his book, but symbolism, on the other hand, totally escapes him.
So, occasionally, when we do see symbolic importance in an event, we have to think of the real-life author (i.e. Mark Haddon) at work. One easy example of this is comes near the book's conclusion, when Mr. Shears arrives at Christopher's house carrying a cardboard box filled with his mother's things (233.112). He climbs out of the cab, throws the box angrily into the front yard, and when it lands, a framed photo of Christopher falls out of the box and the glass smashes onto the grass.
Yep, there's no avoiding it – this is some heavy-handed symbolism. It's like in a horror movie, when the young woman says, "I'm just going to go down to the basement to find a flashlight," and thunder booms overhead, lightning flashing in the window. In this case, the photo smashing can point to any number of things:
What we do know is this moment is not at all unintentional.