Regarding the lettuce, it's a cool-weather annual, so it doesn't "come back" per se. The most logical explanation is that it went to seed last year and re-seeded itself. Lots on annual plants do that if you don't dead-head them (meaning, removing the dead flower heads). My dill and chervil reseed themselves every year, whether I want them to or not!
Yeah, that has to be what it is. I've had some lettuce reseed itself this year, which is pretty unusual, but it happened. I regularly have problems with Dill and Coriander reseeding themselves all over the place.
I have a single carrot that survived the winter this year, it's gotten huge.