I have a 2011 Outback, and just recently I'm starting to hear clicks under the front passenger seat. It sounds like a relay clicking, and it happens regularly every 10 seconds. The clicking happens whether or not the car is running, and whether or not the key is in the ignition. When the car is idling the clicking coincides with the RPMs dropping. If the car sits around for a couple days the battery gets drained to where I need a jump start.

What could be causing this issue?

Mobsters planted bomb.  Get out.

Finding which circuit is causing the slow draw shouldn't be too difficult, set your multimeter to amps and check amp draw at the battery terminals. It should be near zero. If not, start pulling out fuses one by one until the reading drops to zero. Then you can go from there to fix it.

The clicking sound under the seat? Maybe a faulty seat warmer?

And yea check for bombs and cia tracking devices

 

not sure on a '11 but, some newer soobs have a periodic fuel tank pressure test they do. Dunno where that system's control is located.