How would the community know of a sin done unintentionally in Numbers 15?

In Numbers 15:22-24, it talks about the community sinning unintentionally (v22) and the community not being aware of it (v24), how would the community know to give the specific burnt, grain, and drink offering? It's without their knowledge so unless there was a failure of something happening (Achan in Joshua 7), how would anyone know the community had sinned? I understand that a sin committed with or without knowledge of the law is still a sin but other than some outside event occurring or not occurring, how would anyone know? Also, since this is the community's sin, where does the offering come from? Does the community have a flock or herd of animals to choose from for their respective offerings? I assume if it's a personal offering, the animal comes from your flock or herd but that's just my assumption.