Wikipedia-grade non-biologist here, so again, feel free to correct me if I'm wrong.

As far as I'm aware, antibodies don't stick around after they're no longer needed... They're produced by B cells when an infection is detected, and after an initial infection, some B cells that learned how to make effective antibodies stick around in your body as "memory B cells" - these will produce the same antibodies again upon reinfection.

Counting antibodies a few years after a SARS infection doesn't seem like it would tell you much about immunity.