If standard stars fields were observed during the same night you can find the nightly zeropoints (and extinction) at
http://archive.eso.org/qc1/qc1_cgi?action=qc1_browse_table&table=fors2_photometry.

The description of the table entries and definitions can be found at
http://www.eso.org/observing/dfo/quality/FORS2/qc/zeropoints/zeropoints.html.
Due to the method used, the nightly zeropoints become available with a delay of about 4 weeks.