In many otherwise secure and happy children, this fear persists over the years, often in subtle and suppressed ways, like a continuous, barely perceptible drumbeat in the otherwise beautiful symphony of their lives. For the children's sake, we have to assume that the fear - along with normal grief over their early losses - is present even when none of us is conscious of it, so that we can avoid doing anything to aggravate it.
A legacy of loss
It is not hard to understand why an adopted child fears losing you when, for example, you need to return to your jobs and leave him in day care. He already has a conscious memory of a recent loss of one or more caretakers, and of course there was at least one other major loss that he may not consciously remember - the separation from his birth mother. No matter how carefully his former caregivers tried to prepare him, and no matter how much they loved him, he is bound to perceive their disappearance from his life as a desertion, as abandonment that he may somehow have caused. He expects a repetition of what happened in the past, and he may act in ways that seem designed to provoke such a repetition, particularly if there have been multiple separations.
It may take a very long time for him fully to trust his new parents to "be there" for him, no matter how naughty he is, and not to abandon him the way others have done. This lack of trust - sometimes compounded by interruptions in an earlier attachment process - is at the root of the negative "testing" behavior that is so common during an older child's first year or so in his adoptive home.
To complicate matters further, your little one may be locked into "magical thinking," which leads young children to assume that they cause everything that happens to them, including the misfortunes in their lives. If other people have abandoned him in the past, as he perceives it, then there must be something dreadfully wrong with him. His self-esteem will suffer to the extent that he believes he caused others to leave him. Related feelings of guilt, grief, and anger can delay adjustment and may reemerge periodically.
His forever family
The good news is that you, as his adoptive parents, will be giving him a corrective emotional experience, showing him by your words and actions that you are his "forever family." While he may need psychotherapy when his distress is most evident or disrupts the family, there is no better therapy for a child than living with parents who continue to love him and care for him "no matter what." The more you are aware of his separation fears and related fears of not measuring up to a standard that he somehow failed to meet before, the more patient you will be, and the less likely you will be to subject him to unnecessary or sudden separations.
If you realize that the child you adopted three years ago may be terrified of your both flying off for a week or two to pick up his new sister, you will be less likely to leave him behind with even the most adoring of grandparents. You will probably discard this option in favor of taking him with you or having the primary care taking parent stay with him. No amount of reassurance on your part that the plane won't crash will convince him, but his anxiety will dissipate if he's told that he'll travel with you or that one parent will remain at home with him. "Actions speak louder than words."