Typically lean codes are from vacume leaks on our cars. But that is just from my experience.The code for the cat is not going to cause any issues from it being on. But the right way to do it would be to get it deleated. Sometimes the ses light does not come on even when the cat is deleated. Sometimes it comes on after driving for a while. Sometimes none at all. So that may be the issue for the code not being there at the time you checked.