What causes a fever with no other symptoms?

Many things. Extensive physical exercise will generate elevation of body temperatures, and any form of inflammation, whether infectious or not.

Related Questions

What could cause a fever with no other symptoms in my 9 months old and my 3 year old?

Depends... Despite understanding your concern of unexplained fever in your children, this actually is all but impossible to answer without a physical examination and perhaps even labwork. This a medical quandary that most pediatricians face daily, hence your pediatrician would be the best resource to reckon with this dilemma. There is an explanation for the fever--but w/o further input, it is but a guess.

What could cause reoccurring fevers with no other symptoms?

Periodic fevers. There are conditions where a fever may occur on a regular basis time wise-called periodic fever syndromes. These are complicated and you should discuss these with the pediatrician.