I set off just after sunrise as the light levels were increasing and, yes, it was raining. I can't remember when I noticed the full beam symbol but I'm pretty sure it wasn't there when I set off (ie the darkest point of my journey!)
From what you've described, this doesn't sound like the auto high beam. A more likely scenario is that you've accidentally moved the indicator stalk into the high-beam position (ie lifted it towards you,) without realising, which is easy to do when moving the stalk up and down to operate the indicators. Then when the wipers switch the lights on, which happens automatically, they've come on on full beam.