Concentrated hydrochloric acid when kept in open air sometimes produces a cloud of white fumes. The explanation for it is that

Concentrated hydrochloric acid emits strongly smelling HClgas all the time

Oxygen in air reacts with the emitted HClgas to form a cloud of chlorine gas

Strong affinity of HClgas for moisture in air results in forming of droplets of liquid solution which appears like a cloudy smoke

Due to strong affinity for water, concentrated hydrochloric acid pulls moisture of air towards itself. This moisture forms droplets of water and hence the cloud