Lingering Ex

There is something strange about my ex. While three weeks ago I was still very much at a point where I would have returned given a hint, I thoroughly feel now that I have moved on. However, in a strange way, my ex seems to linger – and even look for situations which must be strangely unpleasant for him.
I have seen him a lot recently. But not, as you might expect in a situation which is private (like going to a bar and having a drink together), but rather he tends to phone up and ask if he can join me going out or being with friends. This might, of course, include me talking, flirting or in at least one instance kissing another guy right infront of him. Yet, somehow he seems not to mind – or at least will call back soon after and ask if he can come along again.
While very much in the beginning I was really hoping for a sign from him that maybe he would be jealous, I am now over that point and take him very much as just another person being in the same place with me.
Of course, I wonder: it make me a “bad” person flirting right in front of him? Am I playing with his feelings? Or is he so cool and has equally moved on.
Really – what is going on in his head?!