My Doppler is Making Me Wonder...

So tonight I'm playing with my doppler and pick up baby's heartbeat on the left side at 157-160 and I asked Tony as a joke, "What if i found another heartbeat on here? Would you freak out?" and before he could answer, I moved it to the right side and instantly picked up a heartbeat 153-156. I moved it back and forth so I know the bub wasnt moving from side to side. And they both sounded exactly like a fetal heartbeat, not an artery. SO I'm confused. We have had like 5 ultrasounds and they've only seen one baby, but they haven't looked for another one since the 7 week ultrasound. Could they miss one???? We have the 3D scan tomorrow so I'm going to ask the tech to double check. There must be some logical explanation for it, right?!