Most common reason is the fixed wheel sensors or the rotating 'reluctor ring' getting miss-aligned or damaged. The face of the sensor needs to within a few mm of the 'reluctor ring' which is a posh name for a toothed cog. The sensor is a hall effect type which can get confused if, for instance it gets a bit of greese on it which a flake of ferrus metal has attached itself to. After that check all the wires to the sensors, as they are a bit exposed.
The ABS ECU/Hydraulic block is fairly bullet proof.
You could narrow down the search for the fault if you had access to a Peugeot or Citroen diagnostic tool known as a lexia or peugeot planet.
I'm a member on frenchcarforum.co.uk which has a lot of citroen enthusiasts on it. The peugeot section is often quiet but if you search the whole site, the answers to questions about ABS will be the same as for peugeot
Edit: I should have said, if the warning light comes on only after you start moving, then it's a good indication that all is ok with the ABS ECU and the supply wiring to the sensors, as it can't tell if there is a problem picking up pulses from the gear tooth sensors until it is expecting some pulses. On the other hand, if the light comes on when you first start the car then it might suggest the opposite.