Hiding symbols inside logos can be a gimmick, but it can also be done very well and for good reason. Here are a bunch that span the spectrum. Some are a bit of a reach (the hidden tire in the Continental logo?) but there are a lot of good ones here.
This is where I store random things that I think are neat. Graphic design, web design, industrial design, urban planning, you name it... If it's design-related, I probably like it.