Your not noticing something doesn't invalidate other people noticing them. Maybe their area has different hardness of water than yours does. Maybe they're a supertaster and you're not. Maybe they have pregnancy nose. If they can smell the dishwasher detergent on the microwaved plastic, they'd be able to know that plastic's been heated up with dishwasher detergent.

It seems pretty extreme to jump to saying they're imagining a phenomena you haven't noticed.