The phrase knows where his towel is is roughly equivalent to hoopy frood, but the towel reference is far more well known and commonly used. Thus, hoopy frood is also a loaded slang term for Hitchhikers Guide devotees. The regular geeks -- even the ones that haven't read the entire Hitchhikers Guide series -- know the towel reference, but only hoopy froods know and use the phrase hoopy frood.
I bring it up because: This week And Another Thing..., the final Hitchhikers Guide to the Galaxy novel, went on sale. While it may be the last chapter of H2G2 universe, And Another Thing... is also the first Hitchhiker's Guide novel not authored by the late Douglas Adams. Thus, all the hoopy froods are trying to sass the new author, Artemis Fowl creator Eoin Colfer, to see if he really knows where his towel is, prose-wise. Here's hoping he can bring the funny, and that And Another Thing... isn't the prose equivalent of Vogon poetry.
