According to an insider close to Kate, the reality TV star felt like she had to spy on Jon to protect her family, for fear of what he was doing when she wasn't around.
"The only reason Kate was so strict on the nannies pertaining to Jon is because he tells the tabloids what is going on within the family," the source told RadarOnline.com.
"She also does not allow Jon in her house at all because he takes papers and whatever he can get his hands on to use against her."
Indeed, Kate previously accused Jon of stealing the family's computer hard-drive and giving it to author Robert Hoffman so he could write a tell-all book on her.
Revealed in the secret babysitter rulebook, the jaw-dropping manual gave clear directions to eavesdrop during phone calls between her eight children and their beloved father.
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"ALWAYS listen with one ear to hear what they are saying," she writes. "You are always my eyes and ears and need to report everything you see and hear!!!"
"Kate just wants Jon to live an honest, hardworking life and stop dwelling on their failed marriage which ended 6 years ago," the source said.