The police officer was entitled to stop and question you with respect to a trespass incident well after the incident happened. As to being charged with a crime you very well could be by the district attorney's office based upon the report of the police officer from what you have written.However, proving such charge for trespass and a minor in possession of alcohol (MIP) seems to be an uphill battle for the prosecution from what you have written since you were not caught at the scene nor with alcohol in your possession.