I'm sure that if you asked enough NASA rep's about this, I'm sure that you could find someone to tell you "no, you can't keep it". And for that matter, the same can be said of probably anything we find, anywhere. But seriously now, did anyone really care? Is anyone still out there looking for debri, or thinking to themselves "gee, I wonder if md'rs might find some bolt or screw?". If you asked around enough, I'm sure you'd find someone to say "you can't keep that". But it would probably be an issue of "no one cared, ... TILL you asked" routine.