Encryption and password privacy is an entirely unsettled area of US law. The courts can probably compel you to enter your password (to decrypt a drive, or what have you), while you can maintain that the content of your password can be protected under the 5th. So, for instance, say you had encrypted files of plans to build a bomb and detailed schematics of the White House. The judge can order you to decrypt the files…
How easy would it be for an officer to present an encrypted computer and say - if you don't unlock this, you're getting thrown in jail. This is no different than a witch hunt, it's totally unprovable and throwing people in jail over not knowing a piece of information is unethical.
Yes, there are going to be gray areas. But if Bob has one computer in his house with his and only his finger prints all over it, wear that indicates that the computer has been used extensively, and the computer hard drive is filled with an encryption scheme wrapped around otherwise useless gigabytes of random data, then I am sure beyond a reasonable doubt that there is encrypted data on the hard drive and Bob has the key.