It is safe to assume that all the history would have been removed. You are best to wait and see if the fault re-occurs.
Depending on the fault, the warning light can come on at the first instance of an error, second (two trip logic) or on the third strike (three trip logic). Certain error conditions are only tested for periodically, when certain conditions are met. Here is an example of how one type of emissions fault is detected:
For this type of issue, when efficiency is just starting to drop, it will only be detected intermittently. After clearing the error you may go two or more weeks before you get a warning light again. Hard faults on the other hand are detected straight away and when cleared will immediately reappear.
At this point in time we are just speculating. I'd personally wait until you get another warning - it has been know for a spurious error to occur that never repeats. If there is a problem then you will get another warning at some point. If you take it to a dealer now they will spend time, and your money, trying to recreate an issue that they may not be able to.