The radar will be reflected by the propane tanks which are steel or titanium if the pilot is extremely rich. The pilots watch will also give a good return signal
As has been said above, the return will be filtered out as clutter due to the balloon tooling along at 15kts.
A lot of balloonies avoid talking to ATC if at all possible due to lack of practice and the fact that firing the burners makes it impossible to hear the controller. Not firing the burners is rarely a good idea ! They do (generally) know their position with respect to zone boundaries to a high degree of accuracy. Navigating with a GPS, an OS scale map marked with airspace restrictions and with an unobstructed view of the ground is a piece of cake.