Information. With modern technology, it’s easy to carry a large amount of it in a small space. Or, if it’s something short and simple (bank account numbers and passcodes, say), you could memorise it, or disguise it as something else. Bank account numbers can be made to look like phone numbers in an address book, for instance.

Another possibility is rare stamps. I saw a film a while ago where several people were chasing around a city trying to recover an envelope that was assumed to contain a letter with some important information (or a cheque for a large amount of money). When they eventually got their hands on it, the envelope turned out to be empty. The valuable thing was actually the stamp on it, which was incredibly rare – but anybody who wasn’t an expert philatelist would never have known.