Microwave plasmas are used in industrial, high-quality SiO2 deposition processes for the production of glass fibres. When using chemically reactive process input gases - in this case SiCl4 and O2 - dust particles can be formed during deposition and can be incorporated in the deposited structures. It is known that dust deposition can be substantial at pressures above 30 mbar. In a recent study, a typical poly-disperse dust particle population was found to have radii in the range of 10 to 100 nm. This work focusses on the in-situ detection of dust particles in the gas phase, in an industrial system where SiO2 is deposited on the inner wall of a cylindrical quartz tube. The detection of these particles is performed by means of deploying high-power sub-nanosecond laser backscattering in the visible (532 nm). With this technique, the influence of gas pressure and the temperature of the environment on the particle formation/growth process is explored. Using laser backscattering along the axis of the cylindrical plasma system, the presence of dust is successfully detected directly in the gas phase. As expected, the scattered light intensity increases as a function of pressure. Our experiments reveal that no dust is detected at pressures below 20 mbar, which is above standard deposition conditions. The nanosecond time resolution of the detection system enables spatial resolving of the scattering process, and thus gives insight into the spatial location of the dust with respect to the plasma. The most intense scattered light - which is a function of both particle number density and particle size - originates from about 1 to 2 meters downstream from the plasma, depending on pressure and gas temperature. This indicates that particle growth and/or coagulation continues outside the plasma region. It is furthermore shown that the growth rate of dust particles is independent of gas pressure.