Interestingly enough my wife and I had a similar experience a few years ago. We were talking in bed in the early morning sometime after midnight with all the lights off. At the exact same moment we saw a flash of light as if a camera had gone off in front of our BEDROOM closet door. We lived on the second floor in an apartment in the suburbs. We've seen what cars and "normal" lights look like from our LIVING ROOM and this was nothing like that. I thought of it recently to try and find an explanation and this is what I've found:
1) Migraines and other neural conditions, apparently cause flashes to appear. The problem I have with this is that my wife and I saw it simultaneously. The odds of us seeing the exact same thing at the exact same spot at the exact same time seems too great especially considering we have no medical history of any neural disorders =)
2) Spiritual activity, I'll be honest its not that I don't believe in spiritual phenomenons but this just didn't seem like spiritual activity. At lest not like anything I've ever heard of before from any "experts"
3) Alien abduction, again I do personally believe in extraterrestrial life. However, I'm not sure I believe that E.T. has traveled many light years and arrived on little ol planet Earth yet.
4) Random energy, this is what I really wanted to believe it was but I'm thinking it is very unlikely. When certain atoms collide it can create energy. For example, if anti-matter collided with matter then it would theoretically create a burst of energy that would likely resemble a flash of light (1). Now I'M NO PHYSICIST but I would think that if this did occur the effects would probably be more than harmless. But again that's my layman's mind thinking.
In short, I honestly don't know myself and I'm still trying to find an answer that I can swallow. I know this isn't a definitive answer but it might help you in your search for an answer. As for me I'll continue to searching as well.
(1) A Brief History of Time - Stephen Hawking
Also Google searched various queries such as "Strange flash of light indoors" or "Random flash of light indoors"