Simply because you or someone you know may have NEVER been arrested, detained, or otherwise registered with any U.S. government agency such as the IRS, HHS, SSA etc., doesn't mean that you may not have created a 'paper trail' in the U.S.
Even if you have avoided opening bank accounts, utility services, requesting an individual tax identification number (ITIN), getting a lease under your name etc., it still doesn't mean you haven't created a 'paper trail' in the U.S.
Often times when someone asks "how will the government know that I or someone else is or was here", they don't understand the variety of ways someone in the U.S. can get registered in a national database. The U.S. government has a lot more access to information than the public and attorneys. Although, it is possible for attorneys to do background checks and other kinds of investigations, the government may have more information than what your attorney or professional investigator might be able to find.
Even if an attorney or other investigative professional tells you they didn't find anything, that doesn't mean the government will not find any information about you. People often forget that any time they applied for or obtained a driver's license, state I.D., passport, consular I.D., credencial electoral etc., they got fingerprinted in the U.S. or abroad. Given many intergovernmental agreements, the U.S. government will have access to fingerprint data even if the fingerprints were taken by a foreign government such as the Mexican Consulate.
Also, if you ever got married, divorced, had children, purchased land, home, mobile-home, vehicles etc., all of that information would have been registered with the state or other government agency. All these registration points create geo-footprints that the government can refer to to establish physical presence in the U.S.
Thus, even if you were never arrested or detained by law enforcement or immigration authorities, it is possible that U.S. immigration officials will be able to find out that you have been in the U.S. one or more times.
Keep in mind, as much as you think you or someone else may have entered and lived in the U.S. undetected, given the rampant use of cell phones, social media etc. it is much more likely for the U.S. government to be able to know that you or someone else has been in the U.S. without lawful status through any number of ways.
It is impossible to predict what the government might know, which is why we can never fully answer how the government will know that you or someone else is or has been in the U.S. That's why it is very important to tell your attorney or other authorized immigration professional about your entries and departures to and from the U.S. so that they can provide you with proper legal advice.
If you have any legal questions regarding an immigration matter, please call us to schedule an appointment at 512-443-4788.