The colleague who keeps in touch acknowledges a person beyond the shared timetable.

Most working friendships are held together by proximity, and they end quietly when the proximity does. Someone who stays in contact after the shared project or shift is over is saying that the connection was about you and not only about the schedule that put you in the same room. That message is rarely spoken outright, which is why the occasional note carries as much as it does.

Think of one person you liked working with and no longer see. Send something short with no request attached to it. The point is only to say that they registered with you as a person.