I don't think those testers can ID reverse polarity and ungrounded. Once I start getting funny readings with the three prong tester I pull out the old fashioned 'wiggie' to figure out what's what. The three prong tester is quick and okay as long as everything is right. Once there's a problem I use other testers.

Also, A trick with those voltage sniffers someone showed me was to put your finger over the receptacle openings you're not testing and it will usually prevent you from getting a positive reading from both. Although, the one in that picture is about as far away as I've ever seen a positive reading show up.

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