The people likely to talk to strangers unprompted are also likely to ignore those kind of messages. Anything that can lead to further conversation can be used (like "oh, what's worrying you?") so actually politely nodding and smiling without giving any footing to further conversation works better than being assertive.
> The people likely to talk to strangers unprompted are also likely to ignore those kind of messages.
No, that obviously false. They might be more likely, but they are certainly not likely.