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4. Smuggled out in a diplomatic bag
This is how government's transfer secure documents and goods to and from embassies to their home country's without third-country interference. On the whole, it works well, but only if the host country does not think their diplomatic guests are not abusing the system.
Assange could in theory get inside a diplomatic bag or be posted through diplomatic mail. This is immune from searches, but U.K. police and security services are allowed to scan the items to make sure they're not explosive or in breach of international law.
The bag itself would have to go through a U.K. ports where it is subject to customs checks. If it is suspected that it does not contain legitimate diplomatic content, it could potentially be opened if say thermal heat given off only by a human was detected. At worse, it would be delayed -- held indefinitely -- and prevented from being posted.
Image credit: Sean Michael Ragan.