The OBCZ is situated in a formerly squatted building, now legalized and rented, where you can eat (very, very vegetarian) soup with bread and a simple salad and watch a film every Wednesday evening, all for one euro.
The books can be found in a wooden box on the windowsill.  It looks like this. (Click makes bigger)

We fully expected the box to be empty when we got there. Turns out it wasn't! It was, in fact, full of books.
The books were all stickered on the outside, but not on the inside, and there were no BCIDs to be found anywhere.
Still, we were glad to see the box hadn't been empty all this time.
It seems someone wanted to make a contribution to BookCrossing, but either didn't feel like doing the paperwork, or didn't fully understand the process.
How very sweet of them, though!

We took the books home to register them on the OBCZs account, refilled the box with the books we brought, and were immediately rewarded yet again by someone rushing right over to the books and picking one to take home. It was one that was registered by thegoaliegirl and sent to me in an M-bag. I hope it gets journalled!

The books are now registered and ready to go back into the box.
Thank you very much, BookCrossing pixies. We appreciate it.