This behavior is absolutely normal, don't worry. For example, the old 5500G-EI XRN stacks works with similar levels of CPU and memory consumption even without any port connected. This is why the stack runs internal procedures in order to maintain the whole stack synchronized (L2 and L3 informations and so on).
A firmware upgrade may help to decrease these consumption levels, but this status does not represents any risks to your environment.
HTH