Some cars use "indirect TPMS," which means instead of a sensor in the tire's valve stem, it measures the speed of each wheel and uses some fancy math to determine if the pressure is low.

I'm not sure if the Kia Ceed is one of such cars, but if it is, there may be some wackiness in their indirect TPMS system. Especially considering the OP says it only happens after prolonged driving at high speeds.