Objects of Hefker do not have any Techum, but when a person is Koneh them on Shabbos they adopt his Techum. As the Kollel wrote (Insights to the Daf 45:2), they become "subordinate" to the person who picks them up.
If so, if the first person gives or sells them to a second person, why don't they become subordinate to him and adopt his Techum? Or, more generally, why doesn't an object change its Techum when it changes hands on Shabbos?
S. Siegel
Only an object without any Techum can become subordinate to its owner with regard to Techumim and adopt the owner's Techum. Once the object has a Techum already, it is not subordinate to the person (with regard to Techumim) since it, too, has its own Techum and therefore it does not adopt the new Techum.
If the object is not subordinate to its owner it cannot change its Techum, since the only time an object or person is Koneh Shevisah (= attains a Techum) is Bein ha'Shemashos at the start of Shabbos.
M. KORNFELD