Edge 500 question

When I start the timer after the sattelite search is completed and im ready to go, I usually have to wait before I can go, as there is a red light outside my work. However my Edge 500 registers me as moving? After a minute of waiting at this light, my Garmin said I had moved 100 Feet? Anyone else have this problem?
Cube Attain SL Disc
Giant CRS 2.0
Giant CRS 2.0
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Giant CRS 2.0