This has happened to me too - dont worry - believe it or not its the position of your auto-selector that causes the problem. Normally this fault indication means as the handbook puts it "an incident in the auto transmission". As you say, it re-set when you turned the car off - if there was a fault it would go into limp home mode and contiue to warn you. The first time it happened to me I was scared (expense!!!), the "fault", is actually you being a little too gentle with the lever!
Although in "Drive" the position switch isnt quite engaged and the electronics dont know whats going on so they panic - meanwhile the car drives perfectly normally (except the warning turns you grey). Yours went into limp home mode. It can actually be "made" to show this fault by slowly and very carefully moving from Park into Drive untill the lights alternately flash!
As you know there is no detent lock on this selector, it is released from Park by the footbrake (more usually the detent is locked by the footbrake). However, I would have the transmission fluid level checked (or have it changed), as it may not have been done since the car was new, and its important that it is changed about every three to four years.
Incidentally, you may sometimes have the car stall for no reason when in reverse - again dont panic as I did, its caused by a cable that comes loose - once fixed its done for good, no more probs. Mine was a 206 LX 1.4 Auto. Hope this helps - obviously have it checked, but once I realised what I'd done, I was a little more positive in how I moved the selector and never had it happen again. Its down to the way the selector operates - smooth with small gaps between positions, like the gateless move from N to D and back, more of a little movement than an actual feeling of different positions. Whereas the Fords Ive driven "Click" into every position. Dont be rough, just when you select, move the lever firmly and until it actually stops moving in that selected position if that makes sense, and you feel the resistance in the lever.
Graeme, November 2009