reminds me of testing out a pinewood derby track in my backyard for our cub scouts pack. It had an IR sensor that would detect the cars at the finish line. When someone was standing next to the finish line, it worked flawlessly. Otherwise, it was very flaky and randomly would trigger without the cars trigger it. I pretty quickly surmised it was interference from the direct sunlight, so we put up a pop-up shade over it and it worked flawlessly (without someone standing nearby, coincidentally casting a shadow). The other dads were amazed that I figured it out, but it's just one of those things you learn from experience (and some background knowledge).