OK, so 47 is hired to kill Shaw. 47 has all the tools of his trade and has a week to plan, then a week to execute. Shaw is completely unaware, he is going about his usual day to day activities.
So you're just assuming he has to know he's about to be attacked to absorb anything because it wasn't explicitly stated or shown that he doesn't?
Magneto slammed metal beams into his back. He wasn't aware of those.
He wasn't aware of the bullets being fired into his back in the CIA base.
That's as much as you'll get, but it's clear his absorption is automatic, which is why Xavier had to freeze him, turn off his brain, and Magneto had to ever so slowly force the coin through him, otherwise he would have absorbed it.