Your local parts store can also sell you a perfectly OK basic code scanner for around $60 depending on whether they have one on sale or not. It'll work with any car made for the North American market since 1996 and probably with post 2010 vehicles for at least a few more years. It'll allow you to read and reset codes. It may be less trouble to do the readouts yourself than to drive around to mechanics and parts stores while you work out what is going on. Throw the scanner in the glove box after the problem is cured. Just remember to take it with you when you buy another car.
I don't know enough about ECM programs to be sure. But given that they are computer programs, I'll bet that some of them are capable of checking stored codes, turning the CEL on based on the stored codes, then -- after monitoring the car, clearing the codes but forgetting to turn the CEL off. If your ECM has a bug like that, it could mean that some intermittent condition could give you a CEL on, no codes condition sometimes. You just need to catch the codes when they are present -- which they will be at times.