Hi EJ,
Not sure if I'm going to be much use but do you think it might be 'perpetual spinach'? It might well have survived previous planting and just be doing what it aways does, just grows. Not as delicate as spinach but does the job with a little judicious doctoring with 'indulgent' stuff like cream! Depends what you want to do with the ground. If nothing planned until later, go with it and use it, otherwise oik it out if you have no spinach elsewhere! Hope someone knowledgeable will come along and solve quandry for you ;D