I was asked this question by a fellow writer recently, and honestly, as an agent I feel like this is always an awkward thing to respond to in person.
If I’m told that I previously rejected another project by the person sitting across from me, it doesn’t really help them (why share that I passed previously if you want me to be excited about what I’m reading now?) and it usually just makes for an awkward pause/transition into the critique. I can’t answer why I passed on the spot, because it’s impossible to remember when you read thousands of queries a year, and so there’s nothing helpful I can really share in response. So, just like you wouldn’t tell an agent that you’ve been previously rejected in a query letter, I wouldn’t share it face to face either.
If I’m still considering a query, chances are I haven’t read it yet or I’m just not going to be able to say anything about it offhand because of the volume of queries that are received (and, of course, we don’t want to/can’t spend the critique time searching through my query inbox to see what the status is). So that leads to another awkward pause/transition into the feedback on the current manuscript, and also tells me that you’re currently shopping something that hasn’t been picked up yet. Which, while that’s generally true for a lot of writers that go on to get representation, isn’t something that you would want to put out in that particular moment – just like you would never share in a query letter how many agents you’ve previously queried or how many projects you’ve written/queried before the one you’re querying now.
So in short, your critique moments are precious (and usually there’s only ten to fifteen of them), so I wouldn’t spend the time trying to figure out what’s going on with your previous query or why an earlier one would have been rejected. Let’s talk about what you can do to grow your craft with the manuscript at hand!