What Causes Random Fevers in Children Without Symptoms?

I made a post yesterday about my daughter having a fever that started on thursday and no other symptoms. I took her to our chkd urgent care yesterday to make sure she didn’t have an ear infection. This is the second time in a month that she’s had a random fever no other symptoms. So the dr we saw did a really good work up. She did swabs for flu, strep, checked the ears, all of that was good. Urine sample, chest x-ray and blood work. She went into my daughter chart and seen everything we have been dealing with since my girl was 2 months old, She told me that she’s really concerned that she’s had a fever and no symptoms and that we are on day 4 and this is the second time in a month. They ruled out a virus or infection based off of what they could test for in their office. She told me if she has a fever still by tuesday to go straight to the children’s hospital not any other hospital and plead for them to do additional testing that the urgent care doesn’t have access to. She wants me to call my daughters dr today as well and tell her what’s going on an demand they dig deeper because she suspects my daughter has an autoimmune disorder and that’s what’s causing these random fevers no symptoms. I’m beyond stressed because we have been battling her lymph nodes being enlarged all over but the oncologist thinks they are benign and now these damn fevers. Nobody else is sick in my house, she doesn’t go to day care but now i am afraid to send my other kids to school and them bring a germ home while we are dealing with this unknown.