We can't definitively say *why* this officer did this, since we can't read his mind. We can speculate that 1) he wasn't 100% sure it was drugs when he saw it, so he wants to ensure it is drugs before charging, either out of fairness or because he does not want to be embarrased if it turns out to not be drugs; and/or 2) he is contemplating offering to make one or both of you an offer to become a confidential informant or provide information about a dealer or source to avoid charges--that is, he thinks you could help him bag bigger game. (Those are the likely legal reasons; if the officer is corrupt or criminal, he could be planning to use this information to shake you down or blackmail you.)As to potential liability: if there is evidence to show that it may have been your drugs, either solely or in part (e.g. you and the passenger where sharing them), or that you were knowingly helping your passenger transport drugs, then yes, you would be liable.