Hepatitis b vaccine antibodies: what is the probability your antibodies are too low after just 5 years?

Low. If you received the 3 doses for hepatitis b then you are likely immune.
Very unlikely. Antibody level induced by Hepatitis B Vaccine does not wear down to "too low" in 5 years. There are, however, small segment of individuals who do not develop adequate level of antibody even after 3 doses of hepatitis b vaccine. They are provided another 2-3 doses of same vaccine.