Baffled by Basil

I've written about my love of volunteer plants in the garden before. Miraculous growth of unplanned vegetables is a fun part of gardening
for me.  However, I have not yet experienced volunteers popping up in my greenhouse until now.  Yes, that's a couple basil plants that have popped up next to a tomato seedling in the picture above.

Here we have another magical basil in a totally different tomato tray.  Strangely, these two tomatoes came from two different sources.  This seedling is from a supply of home-harvested seed that I prepared last summer.  The other seedling is from a purchased packet of seeds.  How do I have basil growing in two different trays?

Here are the three answers I could come up with:
1. Basil seeds were in the potting mix.
2. Basil seeds somehow got in my watering pail.
3. Gnomes.

I hope it's gnomes.