Engine fault warning. Half power mode, then all's fine?

[Breaker]

Got in the car this morning. Started up. The car's tickover sounded a bit rough and the revs were "hunting" on tick over. Reversed down the drive and the just drove up the road and the the engine fault warning came up on the lights and the nav screen! Went into low power mode and said to contact dealer. Drove to work, parked up and turned the car off. Re-started, everythings fine??
Been up the road lunch time and still good? Phoned the dealer and they said the car doesn't always keep the fault codes if it was resolved so drive the car more and if it happens again, bring it in.
Only thing I have done to it since Sataurday (it was fine then) was change the battery in the key fob (I have the comfort setting so the battery doesn't re-charge when plugged into dash and the low battery warning came up on the dash). Wonder if it needed to re-code itself? I'll drive home tonight with the key in the dash rather then my pocket and see what happens.

[Maniac]

Can't imagine a key being responsible for an engine issue. Lets hope it was a one off...

[Breaker]

So far it seems OK!