It could mean a few things. Maybe they're still curious about your life. Or perhaps they miss you a little bit and this is their way of staying somewhat connected without actually reaching out.
It depends. If you've moved on completely and don't care about their presence in this small way, then no. But if it makes you uncomfortable or stirs up old feelings, it could be a bit concerning. However, it doesn't mean they have bad intentions. They could just be nosy. You should focus on your own feelings and decide whether you want to allow this or not.
It could mean that they still have some level of interest in your life. Maybe they're just curious about what you're up to now. It doesn't necessarily mean they want to get back together though.
Well, it's hard to say exactly who he is just from this statement. It could be a neighbor who is just being nosy. Or it could be an acquaintance from work or school that has a bit of a curiosity about you. He might also be a family member who is checking in on you in a more modern way, through your stories.