What are possible causes for battery drains when parked for extended periods?

Most cars now have many modules that incorporate "memory" systems that require a constant low voltage supply, including your radio. Most batteries will hold enough charge to last a month without going completely dead. Not sure what you consider "extended period".
If storing for more than a month, you could either put a 2 amp "battery tender" on it or disconnect the battery (but you need to know how to re-arm the security system so when re-connected it will start).

If the battery is going dead in a short time (two weeks or less) have the battery load tested after getting it fully charged. It may be good enough for day to day use but if it's not in really good condition it will get low quickly.
If you have a drain that's unusual, that will require going through the entire electrical system fuse by fuse 'till you find the drain source. At this point it can be so many things that commenting is foolish as testing is the only way to verify anything.