Mystery animal and off until mid-January

We're off for ski vacation until mid-January, but before leaving I wanted to do a post on a mysterious visitor I'm not sure what to make of. Here's the story.

On our front balcony we had a pot of pelargonium geraniums, still halfheartedly blooming in mid-December (click any picture to enlarge, if you like):

Finally it got so cold that I had to remove them, and after cutting off the foliage here's what was hidden underneath:

Four walnuts! How could they possibly have gotten there, and where did they come from? There's not a walnut tree anywhere in the vicinity that I know of, and we are on the fourth floor up. Here's the view straight down:

And here's the view looking (sort of) up:

So what was it: a squirrel? a mouse? a large bird? And how on earth did whatever it was get them there?

I had been thinking of covering the nuts up with straw so as not to deprive the enterprising creature of its winter storage, but three days later I looked again:

Cross my heart and hope to die - nobody in the family removed those nuts. It must have been the original owner. Any ideas?

See you in mid-January!