That's 0.2 watts of power ... possibly a clock?
The D40 uses a bit more than that. It has to maintain power to the stereo unit so the PIN doesn't lock out, the Body Control Module in case you come along and press the remote to get in, plus the anti theft system (NATS). In my car, I also have the electronic rust protection.
Typical starter batteries have around the 50Ah capacity and shouldn't be drawn below 50%, the alternator will keep them at around 75% so you've got 25% or 12.5Ah from when you turn the car off until the time when your battery starts to form sulphate crystals in a dangerous way. At 0.016A per hour, that's 12/0.016=781 hours (about 4.5 weeks) before you start damaging your battery.
So - it's possible that there are other issues at play.
When you start your car, the D22 draws between 500 and 550A from the battery. If it takes you 4 seconds - no, let's say 5 seconds to account for the power to the glow plugs (less than 40A, so we're overestimating power by a small but safe margin) - to start the car, then you're consuming a total of 550A * 5 seconds / 3600 = 0.76Ah. Really, it's not much at all. However, with a spare (in daytime) 20A off your alternator, it will take about 136.8 seconds to recharge it (2 minutes, say).
That's not REALLY going to happen like that, because the regulator holds the voltage down around 14.1-14.4V I've found, and at that voltage batteries take considerably longer to charge. Say 10 minutes.
That means in real terms if you start your car and drive for just 5 minutes then shut it down, you're slowly draining the battery.
With all that said, there's a way to top that up and reverse the issue. Remember back at the start we worked out that your car is drawing 0.2W? Why not get a 1W solar panel and plug it in? End of trouble. In winter you'll get 5 hours at 1W = 5Wh. Your car consumes 0.2*24 = 4.8Wh so the little solar panel puts you just ahead (in the worst of times). Unless it's garaged, of course!
