Lawyers in CA, please answer?

My family had a former babysitter living with us for two years. Prior to living with us, she moved to a different part of the country, and we allowed her to leave about 10 large boxes in a storage unit we were renting. She paid no rent or expenses either for living with us or for the storage. When she returned to California to live with us, I took her to the storage facility and we removed her boxes.

She recently left us - literally snuck her belongings and herself out when we were out of town. She has now contacted me, claiming that I "have possession of her personal property," because she thinks there was one box of hers left in the storage unit. She won't leave me alone about it, but we no longer have the storage unit - it was emptied out months ago (no box of hers was in it - we would have given it to her then!). At the same time, she admitted (in writing) that she mistakenly took some items of mine when she left and is willing to trade them for this box that doesn't exist. I honestly don't have her box, but would very much like the items of mine she took, as they belonged to my grandmother. Any suggestions?