No. The only reason we suspect a person is cheating is because of (a change in their behavior) which we do not approve of or feel uncomfortable with. Odds are if you found out they were NOT cheating but the (behavior remained in place) you would still NOT be "happy". Being unhappy in a relationship/marriage is "enough" of a reason to end it.
On another note if one is considering hiring a private investigator it's clear (they don't trust their mate). Why would anyone want to continue a relationship with someone they don't trust? That should be enough of a reason to walk away.
The purpose of hiring a private investigator is to (satisfy our ego). We want to "prove" we are (right) about our suspicions. This is especially true if our mate has been denying our inquiries. I say, save your money and move on!
Know yourself, Love yourself, and Trust yourself! If it doesn't feel right to you then it's probably not right for you.