How did Prime know where Sentinal was hiding on the Ark. it was under some mechanical floor that he pressed the button... And was it ratchet that said he hide himself away to protect the pillars.?

Also Ratchet said his levels were faint but he didnt have a device can they just sence if something wrong.?

 

Conceivably, the Ark would have been a very special Autobot spacecraft, immediately recognizable, even as Wreckage. It's plausible that Optimus would have known about Sentinel's mission and that he was aboard the Ark, and its possible that Ratchet knew as well. It's also possible that, as a medic, Ratchet has a number of built-in sensors or scanning devices to take readouts on Autobot physiology, or that he's been trained to evaluate their conditions by quick observations the way a human doctor can with a human patient.

Alternatively, they all knew all this stuff so they could keep the plot moving along until the next big explosion.

 

Doesn't Ironhide ask Buzz Aldrin and the others if they checked the Crash Vault?

The Autobots probably presumed Sentinel locked himself in there and the humans never knew about it back then to even look. Thus that is the first place Optimus and Ratchet look when they arrive in the Ark.

 

Didn't Ratchet have something on his wrist?

Any way, Ratchet probably scanned him with same sensors he did Sam with when he said "he wants to  ream: the female". He just put them on a different setting.

 

Well, they entered the Ship and couldn't find him across the bodies, so they just assumed he was elsewhere, they quickly came to the logical conclusion that he was in the "Crash Vault" [as Ironhide helpfully named it] of The Ark.

Besides, when you take a good look at the design of the Ship, you can see that the Crash Vault was placed exactly in the middle of The Ark, a place one could call the shots [like Captain Kirk], a place where a Ship's captain logically would be [there was even a goddamn CHAIR in it].

Besides, as Ratchet implied, it is the safest place in the ship, the perfect place to hide yourself and something important like the Pillars.

He's freaking Ratchet, the off-screen medic of all badassery, he's the same guy who was [somehow] able to smell human mating pheromones while being a goddamn robot with no previous contact to humankind, the dude is a freaking genius.

So, yeah, detecting a dude's life signals with his bare eyes? child's play.

 

Because Optimus obviously knew the ship. Hell, Ironhide knew it had a crash vault. Optimus would've known Sentinel would be in the vault in the event of imminent crash.

Apparent by the fact that he has built-in surgical lasers and could immediately determine Sentinel's Energon levels without even touching him.

 

It is an Autobot ship with an important technology to win the war and Optimus saw it destroyed (severely damaged actually) as it left Cybertron.

Really the Autobot should have had it escorted. It was chased by Decepticon fighters. The Decepticons have a mothereship-fighter doctrine as we've seen.

Course the only evidence of Autobot flight technology seen is the trailer. Jetfire just donated spare parts. But given Optimus' flight proficiency versus Bumblebee's So-So fighter ship skill the Autobots may not have had a fighter doctrine but a jet pack doctrine.

 

Well, the Autobots probably new because it was an Autobot ship. Sort of like how people know (or should know) where emergency exits and rooms are in buildings. Ironhide asks "Well, did you search the crash vault?", followed by embarrassed, quizzical "...Oh. No we didn't"-looks from the astronauts and Mearing. The first men on the moon shouldn't be expected to automatically search for the super secret crash vault in an alien ship. But for an Autobot in an Autobot ship, it's probably the first thing they look for. Or take the example of a bomb shelter. If you stumbled across a bomb shelter after a traumatic event, would it not be logical to search for survivors in a room meant to hold...well, survivors? The Ark crashed. With Sentinel and his pillars in it. Why not check the crash vault?

And the nothing was really said about "hiding" in the crash vault. It's a vault. You protect and guard stuff in vaults. "He locked himself away to guard the pillars." Detecting energon, life readings, energy signatures, etc..

And as for Ratchet "sensing" something's wrong: I wouldn't put it past an advanced alien robot species to have some sort of sensors in their very body. He could "sense" the Decepticons preparing to mobilize, detect Sam's pheromone levels, Starscream said that he could no longer track the Autobot's signals, Megatron can "smell" Sam. (Now, whether or not this "smelling" is literal is up for debate, as both literal and metaphorical explanations make sense)

This. Ratchet's probably got enough skill to do that. Yeah, you don't need a device to hold to detect certain things. However, I can also just picture Ratchet walking up to a dead Autobot corpse on The Ark, staring at it, putting his hands on his hips and proudly declaring: "After much analysis and observation, I've come to the conclusion that these Autobots...are dead." A real genius.  

EDIT: The Ramjet and Balder basically summed up my answers.  

 

Sentinel gave Optimus the manuals of the ark.

 

I think Optimus Prime had a great knowledge on what and who was that ship. The what being the pillars and the who being Sentenil Prime. I think Optimus Prime was after the pillars and not Sentenil Prime before landing on the moon and investagating the Ark. Sentenil Prime was just a bonus find.

 

Optimus knew it already that Sentinel was in the Ark, He said that in the NEST base before he went to the moon.