Do I need to tell the neighborhood Committee to post the cat-finding notice?

Need. It's not illegal to post notices about finding cats in the community, but it's uncivilized. You need to inform the neighborhood Committee or the property, and you can paste it after approval. Telling the neighborhood Committee has another advantage, which can help spread information and is more conducive to finding cats. Neighborhood committees are grass-roots mass self-governing organizations for residents' self-management, self-education and self-service, and they are the important foundation of China people's democratic dictatorship and urban grass-roots political power.