Don't forget to look for the Pizza Planet truck in EVERY movie...Yeah they tend to reference their other works a lot in their movies, including the other films, apparently even Wall-e appears from out of a bin at one split-second around the bit where buzz and woody land in Andy's mum's car.
Outside of Toy Story and Toy Story 2 that is... mainly because it's important to the plot of both movies, and therefore, not hidden.