The other day I was walking through the lobby of an office building and a smell hit me from nowhere. It was a slightly fruity, chemical smell. A bit like shampoo, a bit like perfume. Suddenly I was transported back to my childhood and weekends round my friend's house. His parent's hallway had precisely the same small, or at least one so similar as to kickstart my memory.
I have no idea what the source of the smell was, either then or now, but it brought a smile and happy memories.