I've done exactly what you describe (drive in, camp, leave without breaking the law) in the White Mountains in NH[0].

In general, dispersed camping[1] is legal in almost all national forest and BLM land. I think many people don't realize this but you can camp for free almost anywhere in a national forest (keeping certain distances from trails, roads and bodies of water)

[0]: https://en.wikipedia.org/wiki/White_Mountains_(New_England) [1]: https://en.wikipedia.org/wiki/Dispersed_camping