You're not imagining it, but by all accounts, that scene was never released in the theater. You probably saw some of the early marketing materials (mentioned below) and your brain filled in the rest.
Wookieepedia describes the scene:
Luke Skywalker is in the Tatooine desert repairing a moisture vaporator, assisted by a Treadwell droid, when he notices shining objects in the sky. With his macrobinoculars Luke sees two ships engaged in combat beyond the atmosphere. He jumps into his landspeeder. The malfunctioning Treadwell blows a fuse and is unable to follow. Luke speeds off into the desert to find his friends.
It, or images from it, have been released in the following places:
- Star Wars "Lost Cut"
- Star Wars: From the Adventures of Luke Skywalker 1977 novelization
- The Marvel Comics adaptation
- The Star Wars Storybook
- pre-release publicity photographs, bubblegum cards
- Behind The Magic CD ROM (1998)- silent, monochrome video, degraded quality
- online from a variety of fan websites
- Star Wars Radio Drama (Visual\Hearing only)
- Star Wars (Blu-ray)
The scene can be viewed here.
Supposedly, it was cut because
Before the film was cut, this was the audience's first sight of the young Luke Skywalker, much earlier than in the final cut. It was removed along with subsequent scenes of Luke and his friends in Anchorhead. George Lucas had originally written the scenes and shot them at the suggestion of his industry friends who thought that audiences wouldn't understand the story strictly being told from a droid's point of view.
Upon realizing that the story was really about the droids' adventures and it was them leading things to Luke and Obi-Wan, etc. Lucas took the footage out.