? Did you replace the crush washer ?
I have done enough changes that you can get away with not changing the washer; however, I have had your situation where a slow drip can/will develope.
At under a $1 for a washer from the dealer it is not a bad idea to just change it...
If you did change it , hmmmmmm try again...
Be sure to wipe both matting surfaces so they are clean & torque to 33 ft/lb's for a WRX
you can change it now if you are quick... (do this on a cool car)
Have a drain pan under the car.
Have a new washer ready take out the plug, & put your thumb over the hole... with your free hand change the washer... & put it all back together... wipe up every thing & your self & you're done.