I just listened to the official ABC podcast for 2×18, in which Adam and Eddy clarified that since Greg came to SB expecting to find magic, he was telling the truth when he told Emma he didn’t see anything unusual. As adult Owen, Greg already believes in magic, so it’s not unusual for him when he sees Regina use magic. So, Emma’s lie-dectecting skills haven’t lost their edge since Greg was being truthful.
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"That’s how you know you’ve really got a home. When you leave it, there’s this feeling that you can’t shake. You just miss it." Neal Cassidy