How would you react to this reappearance?

Let’s say—
There’s someone you knew for a while (maybe: a few years, to quite a few,) and maybe knew well. You at least saw them on a daily basis for an extended period of time, in a common skill/craft/profession, and were around them enough to care about them. Maybe you’re their mentor, maybe you just worked with them. They could be a little difficult, but they always were there, on time, and worked fairly hard.
Then, they stop showing up. Really they give no reason or goodbye or acknowledgement, they just disappear.
A year and a half/two years later, they come to an event of yours, entirely unannounced.

How do you respond?—What are you thinking/feeling? What do you do/are you like?

This may come off as a little vague—fill in the details, the context, as you see it. Your response could be anything, just honestly, what would it be?