Can't really pinpoint a specific encounter that's easy to find, however they appear usually near a liberated town, at least from my experience so far. You'll either get a message over the radio like "a rebel soldier is being taken, stop the transport" or an icon will appear, such as "out of gas". The encounters are very minimal, such as dragging a car to the gas to the gas station, or helping a rebel kill 2-3 soldiers, then driving him to safety, which is about a 3-4 seconds in any direction.