Random Moments of Memories

For the third time this week, I have been pre-occupied with a task and a random memory pops into my head. Usually it's been a smell or a certain feel, but the past three times, it has been randomness, no smell, no feel... nothing that relates to my memory in any way shape or form.

I was sitting in my desk earlier working on my notes when out of no where I remembered this random moment where I was walking down a narrow street in Spain, I can't remember the name of the street, but I remembered the buildings were tall, dark, and muggy looking. I snapped out of it within a few seconds and "shook" it off.

What is that? What is that that makes one remember or revert back to a certain time or place in our lives?