This may actually be detectable. If someone who has never, based on their search history, suddenly +1's something that would normally be outside their interest, you could weight that plus one differently than others.
In fact, I would be very surprised if Google _didn't_ weight +1 data, and simply used the total number as their "+1 Signal".
You'd be surprised how seemingly smart and dumb the Google ranking algorithm is. SEO's have been witnessing this for years. Techniques that shouldn't work do, and ones that are extremely complex and tricky are sniffed out by Google engineers quicker than anyone could anticipate.
In fact, I would be very surprised if Google _didn't_ weight +1 data, and simply used the total number as their "+1 Signal".