I wasn't expecting an inquisition in my local Post Office but that's what was happening.

A counter assistant asked each customer in turn what they were posting. The woman in front of me said her parcel contained two pairs of trousers. I was relieved for her that it didn't contain something embarrassing.

By the time it was my turn, I was feeling rebellious. What was I posting?

"A small gift," I said.

What was the gift?

"Why should I tell you?" I replied.

I was beginning to feel like Victoria Meldrew. Why was it the assistant's business? Why should I announce to everyone behind me what I was posting? The assistant looked apologetic and said there were items Royal Mail would not carry so he had to check. I asked for the list and he handed over a pamphlet.

It was illuminating. The prohibited items include: waste, dirt, filth and refuse; human or animal remains; poisons, pesticides, peroxides, lighters, infectious substances, firelighters, explosives, narcotics, car batteries, paint spray and alcohol that is over 70 proof. In other words if it isn't infectious, inflammable, corrosive or explosive you can post it. In other words, there is no need to quiz every customer in public. We could simply be asked to exclude the above. There could be a poster with pictures, as at airport check-in desks.

What rankled most was the invasion of privacy. It seems to have become a national pastime. If I ask for a doctor's appointment, the receptionist wants to know why. If I book a flight, the airline asks the purpose of my trip. I can't buy a pair of socks without the shop assistant requesting my postcode and address. I'm beyond fed up with it. Yet, like all the other human sheep, usually I oblige.

Why are we so obedient? On the one hand, we think there's little harm in handing over apparently insignificant information. On the other, even if we're uncomfortable, we don't want to appear rude to a shop assistant. What's the point of complaining to him or her when they're only following instructions?

Here are some reasons we should refuse to answer such questions: potentially, the information could erode your personal freedom; potentially, it gives unknown others the power to manipulate you or to make decisions about your life opportunities.

You've probably noticed (I have) that, when you open your email account, you find in your inbox tempting advertisements and offers. They're tempting because they are from your favourite shops and they often offer discounts or advance sale alerts. If it is Amazon, the latest book by your favourite author is just one click away.

What's not to like? Sitting behind the convenience is a disturbing process. Think of the fuss when a bunch of newspapers hacked the phones of news-worthy people. Journalists were jailed and The News of the World went to the wall, rightly so. But, given that national outrage, I find it inconsistent we express little or no objection to data collection companies gathering, aggregating and sharing our most personal information.

Imagine the picture that can be built from combining your address, postcode, profession, home valuation, vehicle ownership, supermarket shop, on and off-line purchasing, education level, number of children, viewing and listening habits, credit reports, criminal record, National Insurance number - and on and on.

Who is interested in this stuff? Well, governments, especially with concerns about home-grown terrorism. Who else? Insurance companies, credit card companies, big corporations that can target greater sales with greater knowledge. And criminals, of course. Why do we shred bank statements to prevent identity theft when so much of our information is out there being aggregated and updated all day every day?

Streets and stores have CCTV and I've read about an experimental app that can follow you or me through a shopping centre by tracking our mobile phones. Isn't it anomalous that we struggle to care about the big stuff yet feel encroached upon when the guy in the Post Office sticks his nose into the contents of a parcel?

But the big stuff consists of masses and masses of little stuff. How much it matters will dawn on us when we are turned down, say, for health insurance. By then it's too late to regret the online fitness questionnaire in which you admitted to being a couch potato. When you fail to land a promotion it's too late to understand the cumulative story told by regular photographs on Facebook showing you smoking a fag and sitting in front of a table loaded with alcohol.

To save time and bother we click to accept cookies without understanding that they can track the sites we visit. We agree that we have read the terms and conditions of a download when we haven't. We don't have a clue what we've agreed to share.

A friend who studied larger mammals once told me they rarely look up. It's true of humans. Take a look around the roof level of a familiar street and you will see it afresh. Soldiers are taught to scan upward, it being where snipers or danger lurk.

So will it bother us when we are watched from above? There are already 359 licensed operators of Remotely Piloted Aircraft Systems (RPAS), or drones, in the UK with many more unlicensed users. Drones are an asset to many professions.

Think of assessing the condition of high buildings or giving an overview of river systems and flooding risk; think of tracking migrations or understanding better the rate of ice melt at the poles; think of traffic control; think too of investigative police work. Three UK forces already use drones. In a written statement to a House of Lords committee which met yesterday, the British Airline Pilots Association submitted this: "The public in the UK do not seem to be as concerned about having more CCTV cameras than other countries but if data starts to be collected on individuals using RPAS this may well become a serious issue. There may be a need to introduce specific privacy regulations for RPAS as the majority of rules will have been written when methods of collecting information on people did not include close aerial surveillance."

I hope the public become concerned. And I very much hope that it isn't just regulation around drones and aerial surveillance that is tightened. Many people choose to live in glass houses, both physically and metaphorically. But many more don't. They (and I am in this camp) are just carried along on a combination of technological ignorance and a trusting assumption that things are as they always were in this great democracy of ours; that the protection of personal privacy by law is a given.

But for all the furore and outrage directed at the perpetrators, phone hacking is perhaps less of an intrusion than the aggregate of the data collected on all of us all the time, with our obedient co-operation. The price of liberty is eternal vigilance (of our rights and legal protections). So the next time a shop assistant or receptionist or a form asks you for personal information, I encourage you to refuse or, at least, to ask why.