It was one of my biggest worries whilst I had mine in... mine did not go missing but the second one was a PITA to get out... not sure if it was the doctor's skills in removing it but (TMI alert!!!) there was a bit of my flesh attached to it when it was removed (sorry TMI I know!)

One of the ladies I know who used it had her one go MIA - it was found under US and was effective and then other woman I know had hers go MIA, was located under US but her and her doctor felt it was best to replace it rather than 'trust' that one IYKWIM... I suppose in some cases it still will be effective depending on where it is - and you need to decide if you want to leave it there until its original removal time (how long have you got to go with it??)