It would have been helpful if you had posted the results of a show proc cpu sorted, including, say, the first process lines.
I suspect, however, since you don't know of any CPU process using most of the CPU, it's likely being used within by "interrupt" processing.
For more information, you might review show proc cpu info, in particular, mention of the interrupt level percentage. Interrupt level processing, basically, is "routine" packet forwarding processing, and does not otherwise show as a "process".