I left the company, worked at other places in other languages, the company restructured, moved to India and back, got new premises, headhunted me because someone remembered that I wrote this particular system (serial port controller for certain industrial machinery). When revisiting code that I wrote 15 years ago, I was glad that my comments explained what I was doing, and why, and how, because all the other documentation was long gone. Don’t imagine that the original design documentation will still exist when you come back to it.