Did they miss a twin?!

At my 8 week ultrasound I found out I was having twins, but my dr wanted me to come back the next week because one was measuring about a week smaller and she told me that it was very likely one would be absorbed (vanishing twin syndrome). The next week we went back to find one baby. No bleeding or cramping or anything, just one. It was bittersweet, but we're still thrilled. I'm now 18w5d and am due for my next ultrasound in 2 weeks. But here's the thing: my husband bought me a Doppler and the other day I was listening to the heartbeat. I had an odd feeling that I should move the wand around a little bit more. The Doppler went silent as I moved about 3 inches to the left, and suddenly there was a heartbeat again. I know it wasn't mine- it was high and in the 140's. My husband thinks it was just an echo, but I can't shake the thought that maybe I still have both twins. I've had the same experience with the Doppler each time since. Has anyone else had twins missed on an early ultrasound?