Do editors usually notify agents when a book they've sent over goes to acquisition meetings? Or is it common for them to keep quiet until making an offer (or rejecting)?
In my experience (which is not universal, obvs): maybe. The way an acquisition might go is that you have an editorial meeting, everyone is excited about it, you go around to sales and try to get them excited so you can get sales figures etc, and then you put it on the agenda for the acqs meeting. At the acqs meeting you go round heads of departments and eventually decide if, and how much, you can offer. Then you go back to the agent.
You might well, if you feel it's necessary, tell the agent you're taking it to acqs; you'd feel it's necessary if the meeting is, say, in a week's time, and you're pretty sure you'll be allowed to make an offer, and so the subtext is 'so please don't get sell it in the meantime.' Or you might have been given a deadline for offers by the agent, in which case you might not feel the need to tell them. Or you've had a deadline and you just say 'you'll talk to your colleagues and get back to you.' Or the acqs meeting might be too far off (most places have them fortnightly, I'd guess) and the book too hot, in which case you'd do the rounds of heads of departments informally and make an offer (or a pre-empt) ASAP.
It all kind of depends on the exact circs (the book, the company, what acqs meetings are actually for), and the relationship with the agent. Books get submitted by agents in a detailed, personalised way (if they're any good.) It's an email conversation, not just a manuscript arriving in your inbox.