Sometimes, as IP, you will be asked to locate or identify the physical address of the owner of a PO Box or P.O. Box (PMB). This is often called a Box Break at the Post Office or Reverse PO Box.
This can often be accomplished with a database search if you have access to a database that uses credit headers. When people have a PO Box, they will often use that address for their car bills or payments, utility bills, etc., and the PO Box will appear on their credit headers as an alternate address.
If it is not there, you may be able to obtain the information by contacting the United States Postal Service using a form available on the USPS website. This form requires you to disclose why you need the information. The USPS will only release the information if it is attempting to provide legal service to the appropriate person, such as a subpoena. They even require a case number in court, the case will be tried.
Sometimes these methods will still fail. It could be that the owner of the box has relocated since opening the box and no longer lives at the address you provided to the USPS. It is even possible that the owner of the PO Box used a false identification when opening the PO Box account.
You can try to disguise the owner of PMB by mailing professional-looking material, free offers, maybe even a love letter, anything to trick the owner into revealing your identity. These emails should always include a trap line phone number with call pickup. If the mailbox owner calls, they will get their information and be able to track them that way.
This may seem like a lot of work for a simple PO BOX Break. Establishing trap lines, obtaining credit headers, pretext postcard printing. Many researchers like to outsource this type of work and purchase the information from a researcher who is already set up to routinely obtain this type of information.
There is an old saying that there is no reason to reinvent the wheel. This is especially true when it comes to private investigators who specialize in obtaining information and selling it to other investigators who are involved in a much larger and more complicated investigation.
This would not only apply to a PO box break, it could be a simple location or phone break.