I would suggest the RCS of propane tanks and burners probably have a higher Radar Cross Section than some GA Light Aircraft which are increasingly made out of composite materials.
Controllers may not necessarily notice a balloon because of its low velocity, therefore the history dots wont be easy to interpret and may just consider it as routine radar clutter.
What with compulsory SSR transponder carriage apparently being implemented in 2008 I understand balloons will also have to carry a transponder. Where they get the power from is anyones guess!