Can a twin vanish as early as 6 weeks so the early ultrasound misses it??

I had spotting between 5 to 8 weeks, sometimes even red. ( nothing much , only when I wiped) . Early ultrasound 6 weeks 2 days showed normal fetus with heart beat but a small amount of free fluid in the uterus.

12 weeks scan showed nothing

Could that free fluid be from a vanishing twin??

I'm just asking about the probability or possibility of having a vanishing twin ( due to some other things ).

What u guys think ??