Problem like Nadine had!

I've got to say I don't know what a switching Earth means but the light switch should be a simple switch the same as the others. Earth on a bike or car is the same as Neutral so there is no Earth as such. Basically all you have is Positive and Negative. To try and put it simply the juice goes up the positive to the lights, horn, , coil, whatever, and then when it's 'used' goes back down the negative. However, if there is a leak or a short from a switch, bulb holder, dodgy wire etc, some juice will go straight to neutral and drain the battery needlessly. If it was pulling more amps it should pop a fuse and make it easier to diagnose a fault, if there is one. If your RAC mate has an amp clamp you might be able to pinpoint that 0.4 amps to a particular circuit which could point towards a fault by clamping the main live cable at the battery and pulling fuses systematically until the tester shows no amps are being drawn.

To be honest auto electrics can be a right pain in the bum. I'd get your bike fixing mate to test it and maybe get a haynes manual or look in the owners manual to see what results you should get. It might be that the RAC fella's tester was out and that hopefully the only problem is that the bike hasn't been ridden enough.

If I were you for now I'd try and get out on it at least once a week and do a good twenty miles at normal rev speeds. I've gone through so many batteries on my pointy in the past because of not riding the bike and laziness.

Hope this helps a bit.

I'll agree with Ruffy on this one. It sounds very much like a problem I had with my 650 which turned out to be some chaffing on the loom. Easily checked as you only need to take the fairing off (The hard part is getting the courage to do it, it is quite easy) and inspect the loom for wear.

is that 0.4 amps with the ignition off or with the ignition on, just the bike not running?

If it is the former, then you have a 'short'. As Raf just said, you probably have a exposed wire in the loom that sometimes touches earth/shorts, other times when you park up, it doesn't ... (hence why some weeks it is fine, other weeks it is not).