These things all vary across legal jurisdictions and with what you sign away in your contracts. If your contract doesn't say anything, then yes, your boss opening up your laptop while you're on lunch and reading through your logged-in but password-protected personal e-mail is not just a civil violation but gets into CFAA/wiretapping territory. California has some particularly strong default protections.

But you can 'freely' sign away these rights in your contract. Or if your contract binds you to follow internal policy and it's in the internal policy, that's usually enough notice and consent for the courts. When you're given a work device from a BigCo with a legal department that knows what they're doing, it usually comes with a EULA-style contract you don't read that authorizes everything.

California law does now say you can't be forced to give your employer your personal e-mail or social login, or other way of scanning your personal e-mail or socials. You can't sign this right away (just like you can't sign away your right to a minimum wage or workplace safety), but if you freely sign in while on a company device, network, and time, and your contract or policy is worded so broadly that anything you do on that device/network/time is the company's...