Although dealing with interruptions in remote collaboration has been studied in general, few works have done this for Extended Reality (XR) collaboration. With the current explosion of interest in XR collaboration, we explore the negative impacts of interruptions in synchronous distributed XR environments and propose a novel concept for dealing with them: Absence Agents. We present their requirements analysis, design, and a prototype implementation. We believe that our concept and design of Absence Agents are important for practitioners and researchers alike, as they highlight avenues for future research.