Just found a foul one in DD's gym bag. (Luckily she'd eaten all the pasta salad that had been in it originally but the left behind smudges of mayo had gone mouldy. It's been in her bag for a week!)
I've scrubbed it clean with washing up liquid and hot water and gave it a rinse with boiling water from the kettle for good measure.
AIBU to stick it back in the cupboard and use it again or should it be binned as a health hazard?