It's a change of mind-set from "ticking off a task-list" to "what's happening here?"
Frankly I'm perfectly happy with those who run off or speed through content missing it and having to grind out the difference. It's training. Though that sort of person will probably be downloading a "optimised event sequence" walk-through from the web anyway, so don't really need in-game hints.
On a more serious note the second event should announce itself on the map and on the task bar with the current mechanics, though I'm not certain what the range is. I'd think that would be enough. The person who has wandered off will see there is an event nearby to play and quickly realise it's a continuation.
Is a man not entitled to the hurf of his durf? - Simond
Well the thing is, some events take some time to start after a previous one is finished. People just don't like to stand around waiting and want to get back to playing. Also, some events don't have a continued part too. So, sometimes you wait around and are rewarded with a second part, sometimes you wait around and get nothing.