Usually the filter is mounted between all the return ducting and the furnace itself - potentially it is installed at a strange angle or covered up with tape or something. Here is a picture of mine, it's the darker colored piece the duct is attached to, with foil tape sealing it to the furnace housing. 

[![pic of furnace filter housing][2]][2]


  [1]: http://lh3.ggpht.com/_9Fsn60PATdY/TFIH2AiIT0I/AAAAAAAAAYg/l2zvhXv_AZM/s512/image414.jpg
  [2]: https://i.sstatic.net/2hIn1.png