In this paper we address the problem of detecting faulty behaviors of cooperative mobile agents. A novel decentralized and scalable architecture that can be adopted to realize a monitor of the agents' behavior is proposed. We consider agents which may perform different independent tasks, but cooperate to guarantee the entire system's safety. Agents plan their next actions by following a set of rules which is shared among them. Such rules are decentralized, i.e. they dictate actions that depend only on configurations of neighboring agents. Some agents may not be acting according to this cooperation protocol, due to tampering or spontaneous failure. To detect such misbehaviors we propose a solution where each agent runs a local monitor using only locally available information. The objective of the work is the definition of a basic framework to automatically realize decentralized intrusion detectors for (hybrid) multi-agent systems where interaction is modeled through logical cooperative protocols.