After all, I can only think of a few possible reasons that your head hasn't come back.

One: It was hurt in an accident or something, and is unable to move.

Two: Something similar caused it to lose its senses and become lost.

Two days passed between you realizing the head was missing and you coming to me. You already asked your friends to help you look, correct?

If it were either of those reasons, you might well have found the head yourself. The mice, too, would probably have found at least some sort of lead.

Three: For some unknown reason, the head decided of its own will to abandon you and go into hiding. Then again, in this case the "missing person" is a second head who shares the client's memories and identity, so I doubt it's that either.

And finally, four: Someone kidnapped and took it prisoner.

I'm starting to lean towards this one.