If you are suspected of a crime in which DNA is relevant and set foot in a police station it's not unheard of for them to ask you if you'd like a water in case you drink from it and leave the bottle in the trash.
Sometimes police tail a suspect and wait until they leave their DNA somewhere, like if they drop a cigarette butt on the ground or if they throw a drink cup away.
https://www.policeone.com/legal/articles/8691418-How-surrept...