Bleach
If you didn't add household bleach, also called chlorine bleach or sodium hypochlorite, to your wash load and you discover bleach spots on your sheets and towels, you should investigate whether someone added bleach to the washing machine. Someone may have thought he was being helpful by adding bleach or accidentally spilled it in the washing machine. Always wash items that have been pre-treated with bleach separately to avoid spotting other fabrics. If the washing machine drum was previously cleaned with bleach, and sheets and towels were placed into the drum before the bleach was dry, the bleach could be absorbed, leading to the spotting. Bleach will evaporate when allowed to air-dry; but when it's still wet, bleach can spot your fabrics.
Detergents with Bleach
Chemical technology has allowed detergent manufacturers to create detergents with bleach that protect colored fabrics from staining. Not all detergents with bleach utilize this technology, though, and these are only safe on white fabrics. If you used a detergent with bleach on your colored towels or sheets, you may notice bleach spotting if the detergent does not contain these color-protecting properties. Always read the labels on detergents containing bleach to ensure the product is safe to use on colored fabrics.
Household Cleaners
Many household cleaners, such as bathroom cleaners, contain bleach. Even some dish washing detergents contain bleach. If your sheets, towels or washing machine drum have come in contact with cleaners containing bleach, the fabric is vulnerable to bleach spotting. If you used a towel to wipe up a spill of dish washing detergent with bleach or any other cleaner with a bleach additive, the bleach spots may have occurred before the washing machine was even turned on.
Secondary Exposure
Cleaning any other surface with a cleaning cloth saturated with bleach or a cleaner with a bleach additive, and then tossing the cloth into the washing machine can expose sheets, towels and clothing to bleach. Likewise, a dish cloth saturated with dish washing detergent that contains bleach, when washed with other items, such as towels or sheets, will expose those fabrics to bleach, possibly causing bleach spots. Towels or sheets placed over or under a bleach-saturated cloth are exposed to bleach-laded liquids. The bleach on the cloth will be transferred to any fabric with which the cloth comes into contact. Never wash cleaning cloths, towels or washcloths, exposed to bleach or cleaners containing bleach, with colored fabrics. It is best to wash cleaning cloths in separate loads in hot water to remove any traces of bleach or bacteria.