Engine Malfunction Service Now

[daveym]

My ST is just about to kick over the 10K mark and as I was sitting outside a shop with the engine idling waiting for my daughter about a week ago, I remember turning the A/C off and feeling a noticeable drop in engine revs, once she got into the car i drove off and the error message "Engine Malfunction Service Now" came up on the center screen and my car went into limp mode once i got home i stopped the engine and started it again and everything was back to normal. i had hoped it was a 1 off anomaly and kept my fingers crossed until it happened again a few days later as i started the engine, i turned it off and started it again straight away and all was clear again. I have an OBD2 dongle that is always plugged in to the port as i use my phone as a black-box recorder, I didn't think of looking up what the error code was when it happened at the time but i would have thought the ECU logs all faults and stores them but when i did a scan it just came up with no error codes stored in ECU. Would the fact that the OBD2 is left in all the time be the cause of the problem? I was just wondering if anyone else has come across this warning. Hopefully it is just a software glitch?!