Apple has just released a 2.0 SMC update for those with mid-2013 MacBook Air models, which includes a fix for faster than normal battery drain while lids were closed:
This update is recommended for MacBook Air (Mid 2013) models.
This update addresses an issue which may cause the battery to drain faster than expected when the lid is closed.
Click here to download the SMC v2.0 update. Looks like it should help those affected by this battery issue. Let us know if you see any improvements.