This might be a bit obvious, but...
   Have you checked to make sure that no plants are within chomping reach,
say from the edge of a sofa or something?  We went through a similar episode
with Amelia and it turned out to be the succulents she was snagging on the
sly.  Fortunately my plants are non-poisonous varieties (I don't remember the
complete list of plants; I got the list from my vet).  Some common household
plants are poisonous to varying degrees though, so it is a concern.
 
Regina and Amelia (i stole a tomato _and_ a cucumber right out of the
grocery bag and hid them under the dresser! Hee Hee Dance Dance)
[Posted in FML issue 1211]