absorbing twins..

im 10.5 weeks and i ended up in the hospital last week and after an ultrasound we were told that I appear to be pregnant with twins however one sack appears empty. One doctor talked about absorbing twins in the ER and how one just never developed. which makes sense I've heard of it before. however I'm having to go back for more ultrasounds on Thursday. my family doctor has talked about sometimes with smaller babies the second twin isn't the easiest to find.. which I thought was a little weird. anyway i guess what I'm asking is of anyone has every had experience with this or if it's possible there is still two in there?