I have my debris setting to unlimited and just had a this issue. I have dropped many solid fuel rockets that I use as a booster stage. They almost always have a parachute on them and they never show up as debris. I was just doing a re-entry with Jeb, he was in a capsule with a chute that was connected via a decoupler to the orbital stage that also had a chute. Just after re-entry I split the stages and deployed both chutes. I managed to swap between the 2 pieces as they fell and say both chutes open over water to the east of KSP. the orbital stage had more velocity and ended up furter east. Just before splashdown I was unable to swap back to jeb in the capsule as I got the message that there was no debris close enough and I should use the map to swap. I opened the map and jebs orbit was still there (just) but there was no capsule to swap back to. So I watched the orbital stage land. This time I got the orbital stage as debris but not the capsule and Jeb was listed as MIA. In short I agree, It seems that KSP only tracks the debris that you are watching as is lands / slpashdowns (splashes down? hmm...) It may possibly extend to other items that land / splashdown in a close proximity to the part you are viewing. Does anyone know to sure? or if there is a range? can it be increased?