HBcAg which can't be detected in serum can be detected in hepatocytes , why is it so that they become detectable in hepatocytes?
HBcAg is capsuled by HBsAg and therefore can't be detected, while within the cells it's still free (before assembly of the virus).
The test checks for presence of antibodies (IgM) against HBcAg. It does not try to detect HBsAg anyways. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC154683/