It's most likely in the aspen somewhere, I have the same problem with my corn when I use aspen, usually the smell tips me off. If you're really concerned I would just clean out the tank, usually clumps of aspen form where the snake defecates so you'd be able to find something. If you really want to make sure your snake is having regular bowel movements just replace the shavings with a bedding that can't hide the feces (i.e. newspaper or papertowel).