Rex was right in one sense, Megatron was quite lost in his thoughts as he energon came clean off his hands. Typically things of the trooper's size didn't generally pose any threat, so his whispers and movement go largely unnoticed.

It's only until he finishes and rises to his feet does he get a full view of the entire water hole. A little movement does seem to catch his attention, but closer to how one might pause to look at a lizard that just skittered across the sidewalk.