I once had a leak that was hard to find. I had to clean everything up and get under the car with it running to find the leak (ramps). There are some plates on the back of the motor that has a thin gasket which was leaking. The gasket kit was something like $7 at the stealer and took me 20 minutes to...

Spurious knock sensor codes are common, in all likelihood the code will not come back and to my mind there is not much use in trying to speculate why it happened. Great info from Rick, if it comes back check the wiring like Rick says. The sensors basically do not go bad, but the wiring/connectors do.