No they wouldn't be noticed and they are harmful.. Take for example a bathtub... Fill it with water to 3/4 full. Now every few seconds add a small drop of viniger or something similare in color and weight (aka clear and similare weight as water).... OR even just add a spec of salt.... doesn't have to be same weight even. Trust me, you won't notice a damn thing and no one else would be the wiser if you weren't showing them what you were doing or how it was being contaminated. Even after several hours or even days, the tub would likely have not filled up any noticeable amount more... While what i suggested to add isn't typically harmfull.. there are an abundant supply of very deadly or physically harmful things out there that work on this same exact princeaple.