2005 Grand Marquis

My vehicle if it sets for 2 or 3 weeks the battery runs down. This occurs any time of the year hot or cold, in my garage. The Ford dealer has check it twice with no answer other drive it more often. I have had other vehicles with never this problem. MG

This is kind of borderline some of the newer vehicles draw slightly more power when sitting. I use a battery maintainer on my diesel tractor I use mostly for mowing. Auto Zone and Advanced Auto sell them. OTC7645 is a battery drain tester switch. I noticed one on ebay. But use yahoo search before you buy.
Ask the dealer for permissable drain. Disconnect the battery negative and connect a DVOM between the battery negative terminal and the ground cable.
Have the meter set on amps. Their different for different vehicles and maybe somebody else knows what is permissable.
Note wait until the interior lights shut off before getting the reading.

Permissible draw is .055 amps. Also be sure to make sure all lights are off including the light under the hood. Close door and be sure everything is turned off.