I'm delurking to share my story with you
With my second son (my third pregnancy, I had a miscarriage between the two) I was showing awfully early. I convinced the dr to do a quick ultrasound at my 10 week appt because of how early I wsa showing, and also a strong family history of twins. By my dates, I was 10 weeks exactly.
First thing we saw one obvious baby, plus what looked like an yolk sac. He saw that and said "oh honey, you're not 10 weeks, you're more like 6. The yolk sac disappears somewhere aroudn week 6 when the placenta develops." I told him that I had taken my pregnancy test 6 weeks ago, so I most definitely was at least 10 weeks. So he measured the baby, and it measured a perfect 10 weeks (actually, 10 weeks 3 days). So he shrugged it off as a yolk sac that stayed longer. I specifically asked if it could be a twin, he said there's probably no way to know, and that he'd check again next month and see what was there then.
I went home and started researching twins of different gestational ages, thinking maybe I had ovulated a month later as well, and had a twin at 10 weeks and one at 6 weeks. I found very little information on it, but found lots about vanishing twin.
One month later a second ultrasound showed one healthy baby, no sign of the yolk sac, and the pregnancy continued uneventfully. At birth, the placenta had a "spot" on it that the dr found curious, but he wouldn't speculate what it was. I think it may have been where the twin reabsorbed. Interestingly, my son was born with a chromosomal deletion, and it is not inherited (both my husband and myself were tested after he received his diagnosis) and I've always wondered whether there's any chance it's connected...I haven't found any information to support that though.
So...there you go. Mine was much less suspicious than your's, but I sitll wonder. However, there are many days when I look at Connor and think "thank goodness you're not a twin!!!"