Not everyone realizes this, but whenever you use Siri, Apple's voice-controlled digital assistant, she remembers what you tell her.

How long does she remember? Apple isn't saying. And the American Civil Liberties Union is concerned.

A few years back, pressure from privacy advocates and the European Union forced Google, Yahoo, and Microsoft to clear up exactly what they were doing with search data. Google, for example, can save your web search history for ever, but it gives users the option to turn this off; then searches are partially "anonymized" after 18 months.

But if you search using Siri, that search query could be kept for a very long time. In the Siri's Privacy agreement, Apple says that it deletes your user data as well as the "recent" things that you've said to Siri whenever you slide the Siri switch off. But it keeps the older stuff, and it's not exactly clear how this process works.

"If you turn off Siri, Apple will delete your User Data, as well as your recent voice input data," Apple's privacy statement reads. "Older voice input data that has been disassociated from you may be retained for a period of time to generally improve Siri and other Apple products and services."

So your Siri queries may be stored anonymously by Apple for just a few months. Or they may not.

"It's not clear what 'disassociated' means. It's not clear what 'period of time' means. It's not clear what using it to 'generally improve Siri and other Apple products and services' means," says Nicole Ozer, a lawyer with the ACLU. "The only thing that's clear is we really don't know what may be happening to the personal information we have told Siri, even after we turn Siri off."

This matters, because the things that people say to Siri are often more personal than the search queries they type into Google or Bing. Siri is, after all, a personal digital assistant; someone you can dictate messages to. And if that information is stored on Apple's networks, it could be used by marketers, or by government investigators, or even subpoenaed in a civil suit years later.

And because talking to Siri feels just like talking to a mobile phone, many people don't realize that the software is recording things on faraway servers that they'd prefer to keep private, says Ozer, who blogged about Siri and privacy last year.

"People say very personal information to Siri," she says. "They are encouraged to think of Siri as their confidante and their assistant. But Siri is not just working for you. Siri is collecting a lot of data for Apple and for its business. And people should be very mindful and aware of what's happening to their personal information; how it's being used; and be able to make informed decision about whether or not they want to be sharing certain information with Siri."

What happens with everything that Siri learns is a big enough concern that last year IBM CIO Jeanette Horan told MIT’s Technology Review that she'd banned Siri outright on IBM's networks, worrying that what people said to Siri might be stored somewhere.

It has become a bit easier to find Siri's privacy policy, Ozer says. iPhone users can see it in their phone's settings when they go to switch the voice assistant on or off.

But Ozer still faults Apple for not being clearer about its data retention policies and for making it hard for consumers to find the Siri privacy policy. It is not, for example, linked on Apple's Siri FAQ page. In fact, it doesn't appear to exist anywhere on Apple's website.

"For folks who want to find out about Siri and about its policies before they own a new phone, as of today, I still couldn't find any link to the policy," she said Wednesday.

We reached an Apple spokeswoman via telephone, but she couldn't explain the company's data retention policies.

Google, which offers its own Voice Search software on Android phones, says that it anonymizes audio samples and then stores them for up to 24 months to improve its speech recognition software. "We have no way of telling who spoke any particular query," the company said.