Something else that occurred to me:

Jack/Ianto dynamics are really different now.

Think about it: in their very first scene together, who is flirting/teasing? Ianto. And Jack is very passive, almost uncertain with his adorable little "... Really? :D" And then later, when Ianto is needling him about the exploding head-- the comments are humorous, but very much directed at Jack. And far later, in the odd little antenna scene at the end, Jack is actively fleeing, and I don't think he'd have that reaction to any other team member. (Though I don't think that's a negative moment, more like 'if you're going to scold me, let's do it in private. and then let's make out, plz'...) Ianto seems to be the one dictating their relationship, at least at work. This continues what we saw in KKBB, when in the office scene it is Ianto who gives the orders and Jack who follows them.

If Ianto is realizing that he's allowed to tease the boss at work, maybe he's still working out the limits of that-- testing the boundaries, so to speak?

Or, you know, maybe I'm thinking about this too much. :) Sorry to spam your reaction post.