In other words, if the FIA didn't have any suspicion that McLaren might do something to sabotage Alonso's car, then why send an official to chaperone at all? Why not just trust McLaren?

The answer is not so obvious.

The truth of the matter is that McLaren is not the only organization that wants Hamilton to be champion. The FIA does too, but the FIA can not make it so obvious. The FIA must appear to be impartial and *as if* they are treating all of the drivers fairly. This is why the FIA welcomes all of the bad press from the Hamilton camp suggesting the FIA is for some reason out to get Hamilton. It gives the FIA an alibi should they ever be accused of favoring Hamilton, and the Hamilton camp knows this, so they play along. The fact that the FIA seems to object to anything and everything else, except the criticism towards Hamilton is proof.

And to cover all their tracks, sending a chaperone into McLaren looks like the FIA is coming down extra hard on McLaren, showing a lack of trust, when in reality it provides McLaren with the alibi they need, if and when they do something - what ever it is - to sabotage Alonso's car and someone from the Alonso camp accuses them of doing so.

In other words, who is going to believe McLaren did anything to Alonso's car if the FIA was right their watching?

Pretty clever by the FIA if you ask me.