While watching it (and photographing it), a bright mag. +0.5 flash occurred close to it, followed by another one and than a longer +1.5 flare. It turned out to be the malfunctioned US-French Oceanographic satellite Topex/Poseidon (92-052A), which I have observed flashing before. It malfunctioned in 2006 and since has started to tumble.
Below is the image showing both USA 129 and Topex/Poseidon, the latter showing the three flares:
The image yields this brightness curve, with peaks (two narrow and one broad) spaced 3.6 seconds apart: