Yes - that's why its worth doing a DTC read AT THE TIME.
If there's nothing shown then you can tell the dealer that, if you get codes then you have them just in case they disappear down the track.
And lastly - you can then tell if the dealers spinning you a load of BS - if it says X is playin up and the dealer tells you Y.... then you know they have no idea either
The less information you pass to the dealer the more frustrated both you and they ar going to get.