Mystery Plant: Tall Yellow-Flowered Spike Isn't Chicory

A gardener brought an 8-inch clump from a wildflower mix to a nursery three years ago, labeled as chicory. Now the plants stand five feet tall with spikes of yellow, four-petaled, mildly trumpet-shaped flowers. True chicory (Cichorium intybus) always bears blue flowers, so this is something else.

Without photos it's hard to be certain, but candidates include wild mustard, yellow rocket, or a Brassica relative from the seed mix. Close-up pictures of the flowers, leaves, and stem would allow a definite ID.