You have whatever guarantee the law and/or lawful contract provides. If you keep your money in a bank account (not a deposit box) then you are trusting the bank's guarantee they will not tamper with your balance. In most countries, that's a perfectly reasonable thing to expect.
Obligatory "except in Cypress" comment.
I'm wondering how this compares with snail mail. If I write a letter, hold on to it a few days at my home, mail it, the recipient reads it then holds on to it for some period of time. Except in cases where the sender or recipient voluntarily gives up the information, wouldn't a search warrant be required for any government official to get the info? They must get a warrant to search either house. They can't intercept mail without a warrant either, I believe.
For email, usually a user name and password is required for you to access your email client so that you can read or send emails. That implies, even if the email is transmitted in clear text, that some sort of privacy is expected. Just like wire taps on telephones, you should need a warrant to 'tap' into the net to capture emails as they are sent. Where I'm storing the email is irrelevant; username/password is something like a locked door, or even an unlocked door. Without probable cause, the government can't just walk in.
Well, I don't travel by air much, so I don't have a lot of personal data to go on. Awhile back, I forgot and left a cheap, small pocket knife in my shaving kit. The TSA agent politely told me that I could check my bag. My response was that they could toss the knife. End of issue. OTOH, I have a sibling who travels frequently, and got into a 'situation' with one agent. As the story goes, the other agents nearby seems to be on edge/nervous, as apparently this agent had anger management issues and had had run-ins with other travelers. I'll bet that as with any government agency, getting rid of bad apples could be difficult.
Based upon some of the other responses here, I get the impression that most passenger interactions with TSA agents are uneventful.
"Why can't we ever attempt to solve a problem in this country without having a 'War' on it?" -- Rich Thomson, talk.politics.misc