In Heinlein's —We Also Walk Dogs there is a bowl from the British museum that is used to induce a genius to create antigravity. At one point, he asks if the bowl is stolen property and the General Services people say no.
O’Neil eyed him. ‘It’s stolen property.’
‘You’re mistaken. Nor will you find anyone to take an interest in such a charge. Now about this job—’
But well...was it? And how did they get it out of the museum?
It seems not to have been stolen, but to have been purchased by permission of the person in the planetary government who had the authority to allow it—but done with absolutely no publicity of any sort, especially to avoid the attention of British Parliament who would have objected to the release of the piece. And it was done in return for a lot of money, and attention from the right blonde.
"It will take the Government to get anything loose from the British Museum."
... "You remember the treaty under which Great Britain entered the planetary confederation? ... It comes to this: I doubt if the planetary government can touch anything that belongs to the Museum without asking the British Parliament.
"Why not? Treaty or no treaty, the planetary government is sovereign. That was established in the Brazilian incident."
"Yeah, sure. But it could cause questions to be asked in the House of Commons and that would lead to the one thing Beaumont wants to avoid at all costs—publicity. ... Sance and I had better slide over to England and find out just how tight they have the 'Flower of Forgetfulness' nailed down—and who does the nailing and what his weaknesses are."
Then later:
"We located the guy. His weakness is blondes. ... This will have to be a left-handed job. In the ordinary way, it would take an earthquake to get anything out of that tomb. ... I won't get you in trouble. But it will be expensive."
And finally:
"I wasn't the blonde he was weak for," she explained, "but I found the one he was interested in. ... Sance is having a facsimile integrated. With luck, we'll see you tomorrow."
I've always interpreted the passages quoted in the answer by Wildcard to mean that they found the guy responsible for the security of "The Flower of Forgetfulness" and convinced him (by application of female charms) to steal the real "Flower of Forgetfulness" and put a copy ("we're having a facsimile integrated") in its place. No one in the British government is aware of the swap, and the only person in the museum who does know that there's now a fake in place won't talk.
They didn't physically break in and swipe it, but they certainly don't have permission of the British government and the museum operators. They all believe that "The Flower of Forgetfulness" is still there - and everyone can see it still on display.
This passage shows that the folks at General Services certainly don't have the "Flower" legally:
They showed up the next day, apparently empty handed. ‘Well?’ said Clare, ‘well?’
‘Seal the place up, Jay,’ suggested Francis. ‘Then we’ll talk.’ Clare flipped a switch controlling an interference shield which rendered his office somewhat more private than a coffin. ‘How about it?’ he demanded. ‘Did you get it?’
‘Show it to him, Grace.’
Grace turned her back, fumbled at her clothing for a moment, then turned around and placed it gently on the Chief’s desk.
They've smuggled it in, hidden in Grace's clothing. That's not how you transport priceless antiques that you are legally in posession of.
