I often hand my laptop or iPad to my wife so that she can look at something/use it for a few minutes while sat on the sofa (her computer is a desktop in the other room).

I normally don't bother logging out of whatever sites I'm logged in to.

I'm not too bothered what she sees if she opens up the Facebook tab with me logged in - but I can see that it may be a problem for some.

And I don't think that "hand it over for five minutes to check something" is that uncommon a use-case.

"I normally don't bother logging out of whatever sites I'm logged in to.

I'm not too bothered what she sees if she opens up the Facebook tab with me logged in - but I can see that it may be a problem for some."

That's the crux of it - you're not bothered by it. If you were bothered, surely you'd hit the 'log out' button?

Yes and no ...

"here - look at this interesting article on Wikipedia" ... she starts reading ... ... I get bored and go to make cup of tea, forgetting that I hadn't logged out of Facebook ... ... she finishes article, closes tab and finds herself on my Facebook page ...

It is my fault, but a "clear history" (or at least make my history invisible) function would mean that mistakes like the above will have much less impact

Because they share a computer.

Multiple user accounts.