How to locate an (now) adult adoptee, who was last known to be in foster care again?

More than a decade ago, we adopted a pair of siblings out of the foster care system in Illinois. They had a great many problems, including Reactive Attachment Disorder, and we were finally forced to return them to State custody to save our other children, our marriage, and our very sanity. We were told that they would NOT be considered for replacement in another adoptive home both because of all their problems and because they were both old enough that they legally they must agree to their adoption and neither wanted to be adopted again. So, although we gave up our parental rights to the State, they are still by some technicalities still our children. One of those technicalities, according to our attorney, is that they will still have inheritance rights to our estate unless they are adopted again. One turned 18 already and the other is very close. But we cannot be sure they are no longer in DCFS custody as I've been told DCFS can retain custody of a child until 21 "if they want to". And since neither could have yet completed high school even in the best case scenario - that is, no more getting behind from repeated running away - they may still be in foster care for several years yet. What I hope someone here can tell me is how do I go about locating them when they are both 18 and free from DCFS. We have much to talk about. including possible legacies from grandparents. I'd prefer to go through a third party if at all possible at first as our younger children are still at home and we do not want to endanger them by exposing them to possible further abuse by these two very troubled (and probably worst now after years without a family of their own) kids. But we still love them and pray for them every day and ache to know how they are doing. I can find Social Security numbers on my old tax forms if there is some way to trace them that way? I doubt we'd get very much help from DCFS as the local caseworker lost track of them years ago when they were placed far away. They last we heard was an ambulance and hospital bill that wound up being billed to us through a bizarre set of mistakes on the part of a large health care consortium. On one hand, we want to see them. On the other, we'd like to learn they are both far,far away so that our other children are safe, since at one time there was a genuine threat of the older of them kidnapping the little kids so she "could have all her brothers and sisters together with her". We have moved since we last had contact, but probably not far enough away to be entirely safe from such a threat again. We just want to know they are okay. And it would also be nice to be able to give them some of the thngs they should have from their childhood that the caseworker would never take to them for fear they'd get lost during their repeated placements. How she thought we'd ever be able to get the things to them, I don't know, since they never informed us of moves, even before we surrendered our parental rights. Anyone had a similar experience? I know there are others out there who have had to "give back" adopted children. Ideas? Thank you.