They saw twins for me really easily at 6 weeks, so I would think by 8 they would have a pretty good view of what is in there. They can 'hide' but I don't think it's that common.
However, one of my babies has an anterior placenta and the other is posterior, so I haven't felt more movement like I would have expected to. I wouldn't rule out twins just because you haven't felt much movement, especially at 16 weeks. I hope you find out soon!