Yes you can get airlocks in sealed systems, - when you are filling them from new, or more likely if you have been draining system partly or fully. Perhaps less than open systems though, & if both types of system are piped decently, usually not much bother. Removing air locks is similar to open system by turning other rads etc off, while heating running. Clearing air by opening a valve into a bucket or hose, however, would require system to be repressurised. Sealed systems are closed to the air, so don't absorb & release air like open systems, so don't have air gathering.