That actually looks more like Swiss Chard to me rather than spinach but, regardess that is very little herbivory on your chard and I wouldn't be to concerned.
The culprit was most likely an insect of some sort that happened along.
For now you could spray your spinach with a weekly dose of neem oil or the equivalent soap if you are really concerened but, I wouldn't worry about it.
In the future, plant various types of flowers like Marigolds, baby blue eyes, cosmos and so on around your garden (and around you spinach to attract predators that will prey on the culprits. Also, plant some sunflowers which will attract more beneficial insects and birds that will eat insects as well.
And give the beneficial insects thread a read in the organic forum.
Good Luck and happy gardening