went to the doctor to remove my IUD and found out

I went to the doctor to get my birth control, IUD/Mirana, taken out and the doctor tells me that they couldn't find the strings for it.....now I have to get an ultrasound to find it....I got it put in back in 2015 and I remember the pain of it going in.....can someone explain to me how the strings can disappear after about 2 years????