Heiser Toyota

As I was in for a recall for the replacement of air bag sensors. On the first visit I inquired about the check engine light being on and asked what it would cost for an estimate. As I have a reader at home I was able to see the code and clear it, but it came right back. I was just wondering how today when I picked up my car that the check engine light was off. I drove at least 50 miles today and it never came back. No mention of this was conveyed to me that it was cleared or why.