I've been told by my son's doctor that unless he is having a seizure it will not show up on an EEG. I don't know about the factors that cause epilepsy but there are hundreds of reasons known and unkown for seizures.
There are environmental factors that can increase the onset of an attack. There are many factors that can contribute to seizures though it depends from person to person. Epilepsy is a seizure disorder and as such it can only be figured by multiple seizures. They usually will have you stay for a week or two for continued and constant monitoring. It's really the only way an EEG will catch it unless your lucky, I wasn't, for them to catch on a quick EEG.