Disclaimer: I don't own Transformers, but shotgun Optimus, Prowl and Wheeljack if the chance ever comes along.
I thought this part of the movie was probably the saddest in the entire film (I cried), but paved the way for, in my opinion, the best bit of the film... the triumphant return.
This (very short) one shot was born from the fact that I would have thought the Autobots trusted Sam enough to have told him the plan, therefore they must have known about the watch. And then partway through writing this I read a fic where the author said the novelisation said Sam hinted at the surveillance in the conversation... but I like my way better so oh well.
It was Topspin who noticed it, after discretely scanning Sam when they first met. The mech was one of the Autobot's secret weapons, hiding his genius with technology behind the Wrecker's reputation for violence and unruly behaviour. While not creative like Wheeljack (nicknamed Que by the humans), his knowledge of existing technology was second to none. And so only he was able to distinguish almost undetectable data stream transmitting from Sam's watch as impossible for current human technology, however sophisticated; and furthermore identify it as Decepticon tech.
The information was passed on to Optimus, who correctly assumed an attempt by the Decepticons to learn what the Autobots intended to do next. The absence of Carly at the launch site further suggested Sam was being coerced, with his girlfriend's life under threat.
The final piece of evidence was Sam's behaviour; something the first arrivals at least were well acquainted with. Optimus could see the obvious signs of stress in the boy's features and the white-knuckled grip he had on his arm, as if it was about to attack him. Plus the words he had been ordered to ask, spoken through gritted teeth.
"I have to know how you plan to fight back."
"There is no plan" Optimus had to reply, hating himself as he had to watch the conflicting relief and desperation on Sam's face. He really wished he could tell the boy the truth. Sam was one of the few humans to fully accept the Autobots as people rather than machines and had been a good friend to them despite his tendency towards self-centredness in his search for a normal life/doing something that matters. However the entire human race would be condemned to slavery if the Decepticons caught even a whiff of the Autobot's plan.
But Optimus couldn't leave without trying to give Sam hope, or at least sparking some of that determination that had carried the boy through the dual hells of Mission City and Egypt. Hopefully his words gave some comfort, as they had at other times before.
"You may lose faith in us, but never lose faith in yourself."
Taking his leave Optimus turned back to the Xanthium, allowing Bumblebee to say goodbye to his charge. "Make it quick" he said in passing, knowing that the yellow 'bot would break down and tell Sam the plan if the conversation went on too long. He backed this up with a comm. message a minute or so later, calling Bumblebee away.
They both looked back before entering the ship, sparks aching at the sight of Sam hunched over the railing, shoulders shaking with sobs while Simmons offered futile comfort. Optimus was almost glad he wouldn't see the boy's face when the Decepticons destroyed their ship, as he knew they would.
He could only hope that they would see Sam alive and well upon their return.
Hope it did the scene justice. Please R&R.