Is a solar panel clamped to a battery a drain on the battery at night time?
The voltage on a 12V lead-acid battery will always be higher during the charge state than when it is at rest.  So you can expect to see lower voltages 10 - 20 minutes after charging has stopped.  This doesn't mean you're loosing power, this is just the natural chemical response of a battery after a charge cycle has stopped.  As Optionparty mentioned you have to measure the current to know if charging or discharging is taking place.