Anyone had twins missed on early ultrasound?

Kristen • Momma to one sweet toddler & excitedly awaiting arrival of baby #2! :)
Just curious if anyone has found out on a later ultrasound they are hacing twins.  I went in for my first appt earlier this week (at 8 weeks) and had an ultrasound.  It was weird because the tech was training a new girl and was so focused on showing her trainee how to measure things and hold the wand that I didn't get the impression she was really paying attention to the ultrasound.  Anywho...several people (one of which is an OB nurse!!!) who've seen the US have commented that it looks like another baby is there.  Oddly enough, I do have a high chance of having twins and have had this nagging feeling I am having twins (this is my second pregnancy).  I'm 35 and still breastfeeding my 20-month old son (only to get him to sleep at night), both of which are risk factors for having multiples.  In addition, twins are very common on my mother's side of the family.
I have my next US at 12 weeks, but the baby measured small, so they adjusted age to 7 weeks and now I have to wait 5 weeks for next US.  Just going to be a long wait for next US, so I thought I'd ask to see how common it is that a twin may be missed on an early ultrasound!  :)