Forgot my brain in Poland?

After leaving Poland we spent a weekend in Kaunas, a small but nice city in Lithuania. It was yet another sunny day so we set off to explore the old town. At some point we sat down for a few minutes to look something up in the guidebook, then we continued. At some point I noticed that my bag was funnily light... and the guidebook gone. I had no memory of having left it somewhere or lost it.... By logical thinking I must have left it at the spot where we had been sitting, just putting it down next to me instead of in my bag. So we went back to the spot... the book was not there anymore. I got pretty angry at myself (also because the guidebook was a present to Miguel from some friends, with their signatures inside) and even tried to make it appear by kicking very hard into a wooden board, but with no success :-)
Finally we went to a bookshop which had all possible guidebooks in english (to our positive surprise, as in Poland this would have been close to impossible...) and bought a new guide (which is even more recent than the one we lost).
But - where did I leave my brain??