Disappearing coolant.

Maybe temporarily take the plastic cover off of the bottom of the car. If the coolant is leaking onto that, it might be harder to spot.

 

rough idle could also mean poor mixture (ie. bad injector, plug, ignition coil, poor compression). Pull the spark plugs and look for signs of incomplete combustion or steam cleaning. All 4 should be uniform, the wet or discolored one is the misfiring cylinder. If you can get a bore scope, you can also check for head gasket issues.

 

most likely a very small leak in the head gasket, very common. keep the coolant filled and track down the problem before it blows and kills the engine.

 

A very rough idle on startup, usually after the engine has cooled off, followed by smooth running and some coolant loss is typically the mid point of a coolant into the cylinder head gasket leak. Because the issue will quickly clear there are often no codes at first. Later misfire codes will randomly occur across various cylinders. A dual view borescope inspection can be further proof.

If the engine burns oil, which is also common, a hg problem should be solved with a rebuilt engine and updated pistons and rings. Otherwise a head gasket and rebuilt head is recommended if you can find a shop that will do it. Many won’t because of early repeat hg failures costing them money.

A bad plug, coil or injector will continue and not clear up after a cold start. Codes are common. These won’t cause coolant loss.

If it was idling rough and not clearing up, again without obvious coolant loss, it could still be very early hg failure or a sticky egr valve allowing exhaust flow into the cylinders at idle. A sticking egr is not as common and would throw a P0401.

Combustion gas in the coolant tests, water in the oil, compression and leakdown tests usually won’t detect a Prius hg leak until an advanced stage when white coolant smoke comes out the tailpipe. By then the engine is shaking and rattling with every off cycle such as at stoplights.