i had a catalyst below efficiency threshold warning the other day.

The car has been running well. That morning it had had a decent motorway run, followed by a lot of stop/go idling where I switch off the engine a number of times. Once going again at motorway speeds, I pulled into a rest stop. Five minutes later, headed out on the motorway again and the Check Engine came on. There was no difference in the performance of the car. I stop immediately and check it using the IID. I found the catalyst warning and cleared it. Travelled a LOT more that day, motorway, slow urban and medium suburban. No problems and no return of the warning.
So, ignore it, the car is being quirky.
Or something needs to be seen to. And what would that be?
Thanks in advance.

For each Cat there will be two O2 sensors, one upstream is used for closed loop engine fueling. The second O2 sensor is downstream of the Cat and monitors it's efficiency. Once the Cat has warmed up it compares the readings between the two sensors, if the readings are near identical then Cat is doing nothing and therefore throws up an efficiency error.
There are few ways this error can pop up, one is a dead cat, another is a leak in the exhaust system giving a false reading and finally there could be issues with the O2 sensors themselves.
The video below gives info on testing and diagnosing a dead cat:


I would leave it for now and see if it pops up again. If it does then investigation is required.