Perhaps there was an accident - major juice spill or the like - and girl had been changed into spare clothes whilst dress was put on a radiator to dry? My son came home a couple of times in trousers supplied by the nursery after things like that.
I expect the mother has insisted a note goes home (she probably thinks all the kids are desperate to get hold of her little darling's gorgeous dress ). The nursery staff are probably as bemused as you are and are just hoping this will lead people to label their kids clothes.
do they have a dressing up box? they do at my dds preschool (though to my knowledge they have all gone home dr5essed in the clothes they were expected too but last week someone lost a pushchair, as two families had the same model and there was a mix up so you never know)
Not unusual, my dd came home without knickers during the summer term. I asked her why and she told me she wanted to get a bit of air to her 'bit' and the knickers impeded the air flow up the leg of her shorts. I did search the lost property box without success.