Cells are extraordinary sensors. In many cases, their sensory precision approaches fundamental theoretical bounds. I will describe a fascinating sensory mechanism in cancer metastasis where cells use self-guided chemotaxis to follow fluid flows in the body. We derive the limit that diffusive molecular noise places on the precision of this process. Comparing to experiments, we find that cells operate remarkably close to this limit. We then consider crowded cell populations, typical of the tumor environment. We predict a cell density at which this sensory mechanism fails, which agrees with that observed in experiments. Our work elucidates the theoretical limits of cell sensing and sheds light on the mechanisms of cancer metastasis.