Now here's an even more funny thing about what you said. Brains are incredibly good at filling in details. That's why optical illusions work, and that's why, to kids, shadows might be monsters.
If you pay really close attention to your dreams you'll notice your brain is only doing that.
Have you ever seen something, thought, 'it looks a bit like this thing!' and then the next time you look it looks more like it?
One of the coolest examples is when you find text or mechanical things in your dreams. Text, even if you concentrate really hard, always moves about and turns out to be gobbledegook. And mechanical objects, well, you turn a doorknob or a steering wheel and your brain knows what should happen, but if you look really closely, it's not really working properly, it's simply expectation being followed through.
Text is the biggest clue for me for making me realise I'm dreaming. Sometimes you can have some fun when this happens but you might notice the dream becomes really unstable. You'll see what I mean. Lately though, if I see text shifting around I simply think 'Oh, it's a dream.' and I keep on dreaming normally, going with the flow.