There are only various reasons why your ex would contact you.
First, if your guy is horny (most of the guys are), meeting you to pass you back your necklace could be a chance to see you and trying to lay his hands on your panties. He lets you choose, meet him or mail the necklace to you. He is using these 2 options to tell whether you are sill keen towards him. That's the trick! If you choose meeting him, he will know that you are not over him yet. Then the chances of laying his hands on you is higher.
Second, he regrets breaking up with you and he wants you back but seriously if so, he will be saying he regretted breaking up with you and he wants you back but he did not, he used the necklace as an excuse to meet you . WHAT"S A LOSER!
Third, it could be he REALLY found your necklace and he REALLY wants to return you the necklace BUT why meet in HIS HOUSE?WHY NOT MEET OUTSIDE????????
If you still you still love your bf, you can meet him in the house, provided his parents is around and see what is his response?
If he really wants to get back to you?And you said yes? You observe his behaviour. Does he wants to have sex with you VERY QUICKLY??
BEEN THERE, DONE THAT!!!