An unsolicited CPDLC clearance was accepted by a B777 flight crew and resulted in a track deviation when the FMC failed to fly direct between two waypoints as expected.

Narrative

While in cruise; we received an unsolicited CPDLC clearance message from Seattle Center as follows: Cleared Route Clearance Arrival: GOLDN5 Via Direct N4716.0W13151.0; Direct FULMR; Direct HEMLO; Direct ENI. It was in an 'FMC Uploadable' format; so in accordance with our understanding of SOP; we selected 'LOAD'; looked at it in the box and on the HSI; then executed the FMC change and 'accepted' the clearance. So far; so good. But because the first point was very distant from our current position at the time; we couldn't see small variances in the magenta line. From a 'big picture' perspective--i.e.; large scale--it looked perfectly acceptable. However; we were alarmed when; immediately upon passing FULMR; the airplane started a turn to the left; FULMR completely disappeared from the HSI display (rather than simply turning white as we usually see); and there were some unusual magenta lines showing on the moving map. Compounding the confusion was the fact that we had just contacted Seattle by VHF; and had received radar contact. We had expected the airplane to continue from FULMR essentially directly to ENI. When it turned of its own accord; we took it out of LNAV and thought at first to return it to the magenta line between what we expected to be FULMR and ENI. Only FULMR was gone; and there was now a magenta line between HEMLO and ENI; off to our left and behind us. We put FULMR on the fix page; brought the scale back up to where we could estimate a course between FULMR and ENI; and began to steer back toward that line while contacting center to get a revised clearance direct to ENI. That pretty much ended the episode. In the meantime; however; the aircraft had diverged several miles east of the expected course. We were not made aware of any loss of separation in this case; but it was startling nonetheless. So the questions are.... why did we get the ATC directed clearance via CPDLC without asking for it? And why did it include a point that caused the plane to try to go 'back' to a geographical fix behind us and to our left after we crossed FULMR? Queries to ATC (Seattle Center) at the time indicated that; no; that clearance doesn't make any sense; and we don't have any idea why you received it. Also; why was it sent in an uploadable format that essentially sucked us in to accepting; and loading; a bad clearance? Finally; how could we have caught the error? There was no way we could have recognized the impending issue without BOTH going to the Plan mode of the HSI; AND taking the scale down to a very small range. I suppose we could change SOP to require that pilots go to the plan mode; reduce the scale to the minimum; and step through every waypoint in a new clearance. That would have revealed the problem. But that seems to me to be an impractical task to require of pilots in the middle of the night who should have the right to expect that a clearance being uplinked directly to the FMC is a good clearance. Do we now need to treat every uplinked clearance as suspect? We shouldn't have to do that.