We introduce a generic approach for monitoring multithreaded programs online leveraging existing runtime verification (RV) techniques. In our setting, monitors are deployed to monitor specific threads and only exchange information upon reaching synchronization regions defined by the program itself. They use the opportunity of a lock in the program, to evaluate information across threads. As such, we refer to this approach as opportunistic monitoring. By using the existing synchronization, our approach reduces additional overhead and interference to synchronize at the cost of adding a delay to determine the verdict. We utilize a textbook example of readers-writers to show how opportunistic monitoring is capable of expressing specifications on concurrent regions. We also present a preliminary assessment of the overhead of our approach and compare it to classical monitoring showing that it scales particularly well with the concurrency present in the program.