Is iPhone being remotely accessed?
Most likely what you are seeing is the cached image of the app. Your iPhone takes picture snapshots of apps as they run, specifically so that when you go into the app switcher, you see a graphic that looks like the app. It also refreshes this image from time to time. Most likely, you're just seeing a stale picture.

Also on an unrelated note, with the way iOS works, closing apps the way you are doing has virtually no impact on battery life. Placebo effect possibly, but not much more.