Although official on-the-ground environmental monitoring is absent over much of the world, many people living in these regions observe, manage, and protect their environment. The autonomous monitoring processes associated with these activities are seldom documented and appear poorly recognized by conservation professionals. We identified monitoring activities in three villages in the Mamberamo-Foja region (Mamberamo Regency) of Papua (Indonesian New Guinea). In each village we found evidence that local monitoring contributes to effective protection and deters unregulated exploitation. Although everyone gathers observations and shares information, there are also specific roles. For example, the Ijabait hereditary guardians live at strategic sites where they control access to resource-rich lakes and tributaries along the Tariku River. Often, monitoring is combined with and thus influences other activities: for example, hunting regularly includes areas judged vulnerable to incursions by neighboring communities. We identified various examples of community members intervening to prevent and deter outsiders from exploiting resources within their territories. Enforcement of rules and assessment of resource status also help prevent local overexploitation within the communities. Clearly, local people are effective in protecting large areas in a relatively natural state. We discuss the value of these autonomous monitoring and protection processes, their neglect, and the need for explicit recognition by those concerned about these people and their environments, as well as about conservation. We highlight a potential "tragedy of the unseen sentinels" when effective local protection is undermined not because these local systems are invisible, but because no one recognizes what they see.