Based on my own testing of 12-Volt storage batteries, and assuming there is no drain on your battery except itself, that is, the battery's self-discharge, the rate of discharge you have measured is abnormal. I recommend you re-test by disconnecting all loads and cleaning the top of the battery to remove any material that might be conductive and short-circuiting the battery terminals.
For comparison, I tested a 12-Volt battery for self-discharge over a period of 446-days and found the self discharge was at a rate of
0.000695-Volts/day
This particular battery began the test at 12.78-Volts and 446-days later measured 12.47-Volts. The battery was an AGM battery, which generally show excellent, that is, very low, rates of self-discharge. This same battery was tested after 671-days. Its terminal voltage was still 12.29-Volts. The battery under test was stored indoors in normal room ambient temperature. A battery stored in colder temperatures should have a lower rate of self-discharge. There was no load on the battery and it was not charged in any way during the test period.
I also tested a conventional SLA battery, a flooded cell lead-acid battery in good condition, over a shorter period. It showed a drop of 0.15 volts in 134 days, for a rate of
0.0011194 volts/day
Both of the test batteries were used batteries and both were several years old. Both had been in service before being tested. If interested, you can read more details of the testing at
http://continuouswave.com/ubb/Forum6/HTML/002630.html
and at
http://continuouswave.com/ubb/Forum6/HTML/002130.html