A friendly reminder to do all your general maintenance when you buy a used car!

Yes, that is because water got into two of the spark plug wells. To reduce the likelihood of a repeat occurrence, put some black sealant on the plastic cowl center seam. Water can drip down that seam into the engine compartment and land on the cylinder head cover. If enough water gets there, some may leak past the igniter O-ring seals into the spark plug wells.

 

Sealant on the vertical seam of the plastic cowl, where the two plastic pieces join together. The cowl covers the windshield wiper motor.

 

May I ask where you bought your plugs? About what do they run and is it an easy change? I'm assuming so since it's a Toyota, but you never know. Thank you for the reminder. I've been lazy about doing my due diligence with my new-t0-me Prius.

 

OP says he bought the plugs from NAPA. The correct iridium spark plugs tend to cost ~$10 each.

Amazon offers the correct Denso plug for $8 plus shipping here:


It is reasonably easy to change the plugs. Remove the relay box mounted on the cowl for improved access to the spark igniters.

Here is one thread which provides more info:
spark plug replacement | PriusChat

 

You say the plugs look like they have done all of the 94,000 miles on them this is possibly because the change interval for the plugs is 120,000 miles, so they should have been ok for another 25,000 miles. Having said that there is no harm in doing them early.
Possibly the best preventive maintenance you can do is to change the transmission fluid (oil). This is meant to be a lifetime fill, but most here on PC have fund it wise to change at 30,000, and then every 60,000 miles. Only use the Toyota OEM fluid because the motor/generators run in this fluid at high voltages and the wrong fluid could damage the insulation on the windings.

John (Britprius)

 

There is a drain plug on the front of the transmission case with a fill plug above it (no pan). Slacken the fill plug first just in case it will not shift. Otherwise you drain the fluid to find you cannot refill. There is a how to here somewhere I will see if I can find it.

John (Britprius)

 

You only need a 12mm hex key and 24mm socket, and 2ft clear plastic tube to fit on a funnel for refill.

John (Britprius)

 

Still looking for the walk through. You will need 4 us quarts of the Toyota WS fluid for the refill, but it will not quite take all of this. Be aware there is a drain plug for the transmission coolant close by.

John (Britprus)

 

I recall that 10 mm is the right size.

When filling the 4 quarts of ATF-WS, a funnel is much easier than a hand pump.