Service Engine Light Flashing in 2003 Grand AM

Alright, found out that a coil pack was not replaced as first thought. Had the code read and it was a misfire on cylinder 1. Replaced the coil pack and it is running fine right now. However, mechanic says that his scanner is showing that it is still misfiring on the other cylinders. However, the misfires must be so subtle that it isn't even making it to my computer to leave a code and also put the service engine light on.
Mechanic drove car for about 30 minutes and approximately 20 miles or so. He said it ran smooth while he was driving it despite the misfires his computer had picked up. I drove it a good 30+ minutes myself after it had been sitting for a good 4hrs and it ran smooth for me with no hint of any misfires. So not sure what is up with it. I talked to a relative who use to work on cars for Good Year and he had never heard of misfires not making it to the car's computer and he told me if it was running fine to keep driving it until something shows up if at all. Alright, found out that a coil pack was not replaced as first thought. Had the code read and it was a misfire on cylinder 1. Replaced the coil pack and it is running fine right now. However, mechanic says that his scanner is showing that it is still misfiring on the other cylinders. However, the misfires must be so subtle that it isn't even making it to my computer to leave a code and also put the service engine light on. Mechanic drove car for about 30 minutes and approximately 20 miles or so. He said it ran smooth while he was driving it despite the misfires his computer had picked up. I drove it a good 30+ minutes myself after it had been sitting for a good 4hrs and it ran smooth for me with no hint of any misfires. So not sure what is up with it. I talked to a relative who use to work on cars for Good Year and he had never heard of misfires not making it to the car's computer and he told me if it was running fine to keep driving it until something shows up if at all.