While these are all sensible options (although a solicitor would probably raise an eyebrow at keeping hold of a fake passport!), I really need the baddie and a newspaper journalist be able to plant/take something in the recepticle without it getting back to the actual owner.

Yes, he did, and apparently it turns out it's easier to hide an arms cache safely than a bit of money and some paperwork! But I also need the stashplace to be instantly recognisable when the owner does find out it's been used - and also access to be highly restricted (but not necessarily need to know)