Indeed, one of my bigger customers now was in a junior role at a company I did some work for years ago. You never really know where opportunities might lie and it costs little to keep in touch.

I think people also get too shy about reaching out to people they haven't been in touch with. Most people aren't going to be annoyed if you say, "Hey, long time no see. How have you been? Wondering if you know anyone who's hiring" or whatever it is you're asking for.