p0455 evaporative emission system leak detected (gross leak/no flow)

truck threw this two days ago. two days after cleaning my throttle body. any ideas? any tips?

thanks.

http://www.obd-codes.com/p0455

 

Forget to put your gas cap back on?

 

nope. plus i didnt fill up within any of these days. i dont see anything under the hood or fuel filter. i did take the cap off and put it back on. cleared the code. hasnt come back yet. it took about two days to come back the last itme i cleared it though.