What is it doing at this time of year?

I found this caterpillar in the upturned abalone shell which I use as a snail trap and empty each morning.  I have no idea what moth/butterfly it will turn into - perhaps you do?

Update:  this is probably an Angle Shades Moth, for they have two broods a year and one overwinters. If the temperature goes above 5 degrees at night, they come out to feed.