Do people always leave fingerprints when they touch something?
No. Many factors determine whether or not a latent print will be of sufficient quality for comparison. These factors include; the condition of the surface, the presence of a substance on the friction skin, environmental and physical conditions the latent is subjected to after deposition, and the nature of the initial touch. For example, if the surface is very dirty, or the hands are very clean, or the surface is exposed to rain, or the finger is wiped across the surface, the print may be non-existent or wiped or washed away. Conversely, if the surface is dry and smooth, the hands are very sweaty, the initial touch is direct, and the latent is protected until examined, the latent print may contain many clear and visible ridges for comparison.
