Recent advances in low cost spying technology have created a growth industry in spying on spouses by their partners who believe that someone may be unfaithful.
There is an excellent article in Saturday's Wall Street Journal regarding the use of satellite trackers and hidden camera to conduct "domestic surveillance". This was an area that not long ago was the stuff of Cold War spy novels. A small satellite tracking device can be placed in a spouse's car and someone else can monitor a computer program and spy on their spouse in real time. There are common computer programs that can be secretly loaded on a computer that will track everything that is being viewed and typed in the computer. The modern world leaves too many "breadcrumbs" for people who are attempting to have a secret affair.
Obviously learning that someone has been conducting an affair could lead to an immediate divorce case being filed.
There are two Divorce Law issues that should be addressed here. First, a spouse considering spying on their partner needs to be careful not to violate any relevant State of Federal laws regarding wiretapping and illegally recording phone calls without permission. The more import fact to keep in mind is that while the information may lead to a divorce and custody case, there may not be much legally significant information that would be relevant to the Court. California is a No Fault Divorce state. The Court will not take into consideration the fact that a spouse was unfaithful.
One of the few legal uses of spying on a spouse may be the ability to show to the Judge that the other parent may have a substance abuse problem and are drinking or using drugs around the children. This is clearly a serious matter and a Judge would want to be made aware of it.
If anyone is interested in this subject, I would recommend reading the excellent Wall Street Journal article by Steve Elder and Jennifer Valentino-Devries.