Some more Guess/Hint culture suggestions.

Consider:

B: working in the lab now
A: (suspecting, as you did, that perhaps B didn't want to chat) oh ok. give me a buzz when you're free?

This will typically communicate that you've understood that they're busy and don't want to chat, that you're OK with that, and that you want to talk to them.

That said, there exist Guess/Hint cultures in which it also communicates that you have something urgent to talk about, because if you didn't you would instead have said:

B: working in the lab now
A: oh, ok. bye!

...which in those cultures will communicate that the ball is in their court. (This depends on an implicit understanding that it is NOT OK to leave messages unresponded to, even if they don't explicitly request a response, so they are now obligated to contact you next... but since you didn't explicitly mention it (which would have suggested urgency) they are expected to know that they can do so when it's convenient for them.

EDIT: All of that being said, my inner Hint-culture native also wants to add that being visible in an online chat forum when I'm not free to chat is rude in the first place.