Hi, Tierra, Basil, being an annual, is not generally recognized as a plant that self sows from seed; the seeds would normally die in the cold/freeze of winter. Apparently the Thai variety is the most likely to succeed in surviving in a mild climate.
Did you plant Thai basil? If last year's crop flowered and the flowers dried up and, presumably, fell to the ground before you cleared the garden, then these might be basil. You might let a couple go/grow for a while and see what they become.
Or, clear the lot now and . To avoid the risk next year, if indeed it is basil, harvest before the plants flower.