Stupidity: Immediately after Unkar Plutt makes Rey a generous offer for BB-8 and she refuses, he tells someone over his communicator to follow her and bring back the droid. He should have at least waited until she was out of earshot before he said that, especially if he was trying to be covert about it.
Suggested correction: But he was out of earshot of her.
I posted this immediately after watching the scene in question, and it looked as though Plutt spoke into his communicator right after Rey turned around and went on her way; certainly too soon for her to have gotten more than a few steps away. Also, he spoke at a normal, conversational volume rather than anything that sounded like a whisper or "sotto voce", so unless he was relying on the ambient noise of other nearby activity, I still believe this "stupidity" is valid.
That depends on how sneaky you think Plutt is. Rey walks away in quick paces, so she is out of earshot. Also I don't think it bothers him that much if she heard, she is just a scavenger, what can she do about it?
Rey was definitely out of earshot. Rey walks completely out of the shot, which appears to be about 10 feet away. Unkar Plutt then angrily swipes the portions off the counter and picks up his communicator. At the pace Rey was walking she would have been a considerable distance away from him when he spoke. In addition, Unkar Plutt lowered his voice when he spoke because he was being sneaky. She might have been able to hear him speak, but it is totally reasonable that she wouldn't be close enough to make out exactly what he was saying.