|556 Pulse oscillator|
Google told me there was a CMOS-based low power version of the chip, TLC556. That chip was pin compatible and used up 0.3mA. This gave a run time of about 80 days which I was very happy with.
The circuit simply gave a short, loud beep every few minutes. I hid it in my colleagues flower pot and it took him quite a while to find it.. ;)