i think all the levls have got secret areas.....
foundry- pull all switches to open up the slidy bit ( as u said)
canada- grind switch that lets you ge to secret tape...
rio- smash through wall to get to bowl
trailer park - collect axe, and get in theecret bit
airport - go right at the beginning, through the thing that you put luggage through
skater island- boat
la- grind some things, cantreally explainit, but grills fall off and you get a half pipe ( near where pho guy is)
tokyo - avnt got it yet, but you have t grind round wires, soething happs and the big round sign moves (ive heard)
boat- empty pool by grinding valve
hope that helps i may of missed some out, or that the secre areas aint really that great!!