This happened a few weeks ago. My reliable 1999 406Hdi 110BHP developed a niggle after new-year. The indicators would flash a few times after turning the ignition to accessory, then go off when I started the car. This strange behaviour got worse and worse, sometimes having to turn the indicator on to turn it off again.
After buying the Haynes manual (pretty useless unless you want to dismantle the engine). Convinced it was some earthing problem I located the earth points up under the dash in each foot well, tightened but no real effect.
It all went really bad one day driving 40 miles home down the M3. The indicator started flashing (left) and no matter what I did could not stop it, save occasionally holding the stalk hard in and odd position. Caused real havoc with other motorists and I was a complete stress head by the time I got home.
Eventually found the control unit, but forget taking it out, or even removing the connectors, as its behind the fusebox tucked up under the drivers side foot well.
Eventually took the indicator switch itself off and here I found the problem. The indicator switch is just a changeover switch with a centre off position. The off resistance was less than 500Ohms, and this was causing the electronics to false trigger. I could move the switch around and get varying resistances, but never more than a few hundred ohms. For those non electrically minded people, it should be hundreds of MegaOhms, not hundreds of ohms, on an open circuit. I was tempted to take the switch apart, but a) it looks expensive and b) looked like there were a lot of pieces in it, as its the light controls as well. A quick blast through with some switch cleaner, and now the contacts measure open-circuit, and it's all been OK since. Phew!
This is definitely a case of modern electronics fouling up. If it were an old lamp flasher this would never happen.
Robert Older and wiser