Twelfth house planets are a bit of a mystery. There are various theories about what this house really means. The old astrology books talk about this - the 12th house - as the house of downfall. When I was learning astrology my tutors sometimes described planets placed here as representing things or people lost to you - or lost to the person whose chart we were looking at. The twelfth/12th house has also been labelled the house of dreams. To an extent all of this rings true to me. Perhaps most of all, though, this resonates for me as one of the houses of secrets in the birth chart.
Everyone has a few secrets and if astrology can hint at them, it is usually thought to do so through three particular houses in the chart: the 4th, 8th and 12th houses. The 4th house often relates to family secrets, the 8th house to financial and sexual secrets and the 12th house to secrets we even keep from ourselves. You may want to think of it as the house of denial, because it is said to represent the unconscious mind and it is the contents of this part of the mind that we can often deny and ignore - those details only filtering through in dreams and passing thoughts, odd coincidences and so forth.
I think the 12th house is a bridge house - a house that really clearly sits at the edge of a doorway, the doorway to the Self, which arrives and shows itself more fully in the 1st house. So, what happens when a planet is moving through the 12th house by transit - ie, when a planet currently in the sky is moving through the 12th house area in the birth chart? I think that something so secret that it is virtually hidden from the self and the conscious mind may be trying to bubble through to the surface. This may be especially appropriate if there was a planet already in the 12th house of the chart at the time of birth. But it might be significant in the way I've described, even if that is not the case.
As an example, Venus transiting the 12th house might hint at a love affair or secret crush. Maybe the person does not yet realise that they are very drawn to someone. Or maybe they have a clue about an attraction but don't yet want to fully face it, because it would make their life very difficult (remember the theme of downfall!). To some extent this may be a painful house, a battle ground area where the unconscious mind and conscious mind are trying to connect. If you have a planet moving through this house over time, perhaps something that makes much more sense of feelings and intuitions will emerge when the planet enters into the 1st house.
It would be great to hear of your experiences of the twelfth house and planets therein, or planets transiting or progressing through this house!