Seeking maintenance recommendations for 2012 Prius five

We are seeking the wisdom of the crowd on an issue with our 2012 Prius five.

Our goal: We've found it to be the perfect car for us, and we are willing to invest in it to maintain the car in excellent working order for as long as possible (we actually prefer it to newer models). Among other praiseworthy features, the design is such that I at 6'5" can comfortably sit without my head hitting the roof (which it does on all the newer Toyota models we've tried), and my wife despite being very short has excellent visibility in each direction thanks to the older window design. It is a unicorn in this regard.

Relevant stats: The car is presently used for a short distance approximately 5 days per week (10-20 miles), and a longer distance (~900 miles) three or so times a year. It has just over 200,000 miles on it total, has never been in an accident, and it has had all recommended maintenance on schedule over the entire life of the car (we have owned it since it was new). It had a new hybrid battery system installed at 140,000 miles under warranty, and has recently had new brake pads and struts/shocks installed, as well as the engine treatment and fuel injection services done, in addition to the 200,000 mile service.

Here is the issue: a few weeks ago, when starting the car to go into work first thing in the morning (last driven the afternoon prior), the engine made a loud, very dramatic knocking sound for 10-15 seconds, before the sound quieted and went away. This all happened while the car was turned on but still stationary in the driveway. It has not been replicated since.

We took the car to our local dealership and asked them to check to see if they could identify the cause — not being able to replicate the sound, of course, made this more difficult. We did ask them whether it would be a good idea to flush the EGR system and clean the intake manifold, and the answer was a shrug from the guy they had talk to us about it. Here, however, is what the tech wrote in the report (we were not given the opportunity to talk to him, and only saw this on the way out the door):

"No engine or hybrid codes; removed wiper cowl assembly; removed all spark plugs; pressurized cooling system, boro-scoped all 4 cylinders, no internal coolant leaks seen. Performed cylinder leak down test on all cylinders. No pressure rise in cooling system, will recheck again cold but may have EGR [sic], carbon deposits (noticed excessive carbon/soot residue in tailpipe) injector or other issues -- cannot confirm."

They also found evidence of general mouse activity in the engine liner and recommended we use a spray to discourage them.

The dealership person we *were* able to talk to offered us no advice as to how to move forward when asked, nor did they suggest any preventive maintenance when we asked specifically about it. The response was only to tell us to return if the engine knocking happens again. We, however, would prefer to prevent it from happening again.

Our question for you all is: if your goal were to maintain this car in good working order moving forward, and you were willing to invest to do so, what would your next steps be?

Thank you!