how can a body withstand this

For the last two weeks, Sirius had the sense he was being watched. Not in the way he felt just before he was cornered by Evan Rosier and Corban Yaxley in a sideway alley -in Muggle London, no less. No. This felt… Wary, somehow. (Canon divergence where Regulus Black realises there are more than one Horcrux.)