I work nights 2-3x/week and he does this at least 1x/week. The first time, I asked him why he did this and he told me he forgot to put it away. I've tried everything from calmly telling him that it's a waste of my time and our money throwing out dinner to nagging and yelling.
Instead of making dinner this week, I'm going to tell him he can figure it out.
Re: DH keeps leaving dinner out when I go to work
That would drive me nuts!! My DH, instead will put food away, but leave all the dishes for me. It doesn't bother him to see a sink full of dishes, so he'll leave them. Even on nights when he cleans the kitchen, he'll wait until right before he goes to bed to do it (which also drives me nuts!)