If your tests came out normal, that is great news, I think.
The next thing I'd say to think about is how are your lifestyle habits? (you don't have to answer but) how do you eat, do you exercise, do you manage stress well?
Sometimes there are multiple environmental triggers that can cause a migraine. I know you are not getting the actual headache pain, but perhaps something is triggering this in you (and oftentimes its not just one factor but multiple factors together that lead up to a migraine). You could use this experience as an opportunity to go through a health check list and take good care of yourself.
For instance, for years I was a lucky gal and I could eat and drink almost anything with little to no consequences. But as I've gotten older, my body has begun to react awfully to all sorts of additives and unhealthy preservatives and chemicals in foods. I had no reason to prevent consuming them when I didn't feel their affects, but now I'm forced to. I now wonder if some of these same chemicals are making others out there get headaches and slight neurological effects without realizing it. The affects on me are too drastic not to notice. I know that I'm an odd ball, but you never know what could be triggering these episodes. Have you considered making a diary of your diet, sleep, and daily routine? You might be able to figure out a common pattern of what precedes the auras. Just an idea.
Okay, this is rather embarrassing, but I want to give an example: I figured out my trigger. I get the migraine with aura only after being intimate with my committed partner and not using "protection". It took me a while to figure out but I began to see a pattern. My thinking is that the slight difference in hormone balance affects me and instigates the aura and migraine process. Sorry if that is too intimate of information to share but I just want to show that in some cases its possible to find a trigger... and it may differ person to person.
I only share that to say that there could be something in your environment that might trigger the aura for you. Could be a perfume, a food, a stressful event, hormonal fluctuation, abnormal amount of sleep, a nap... who knows. If you watch you might be able to pin point it and then change your habits to prevent the episodes or make them less frequent.
But if that is the only symptom you are having and your tests come out normal, I think that's all good news. :)