Most devices have some form of private browsing mode, whether they call it a private window or incognito mode. The aim is the same, to prevent others from seeing what you are doing.
The problem is, that a lot of people misunderstand how it works. They think it gives them more security than it does.
It’s meant for family, friends and those at work, not the police
If you are looking for a surprise birthday present for your partner, you might use a private browsing window. If they turn on the computer you share or pick up your phone, they will not be able to see that you were shopping at a particular site, or Googling a particular item.
Similar might apply if you are considering a divorce, but are not sure and are not ready to talk about it with your spouse just yet. Keeping your browsing hidden allows you to investigate without them discovering.
Law enforcement, however, is a different matter. If they want to see what you have been looking at and what sites you have been on, they probably can. It might take them a little time, but they can probably get there in the end.
The authorities can search without your permission
The police can get a search warrant to search your phone or computer. They have specialists who can probably access your history even when you go private.
Alternatively, the police can circumvent you altogether. If for example, they want to look at your Instagram or Facebook, they can get a court order and go straight to Meta and demand they hand over records of your activity. If they want to access your browser history, they can do the same with your internet service provider.
If you face an investigation over your internet activity, seeking help to understand your options is wise.