Is this a red flag?

I'm a little confused. Are you asking if not having sales to publishers who provide advances is a red flag? Yes, it can be.

It could mean the agent is new and doesn't have many contacts.
It could mean the contacts the agent does have in advance paying publishers are not interested in what she's presented and she has wound up selling books to much smaller, no-advance publishers.
It could mean she sends out queries/pitches to any publisher who might possibly be interested in the MSs she represents, and then she pushes her clients to accept whatever contract they are offered because a little sale is still better than no sale.

Romance and Erotic Romance seem to have more small and mid-sized presses that an author can submit to without an agent. Even some of the larger houses have imprints that don't require an agent. This is (somewhat) different than genres like Science Fiction and Fantasy where there are only a few that will take unagented submissions and the slush pile is slooooow. Frequently in the 12 month range for a basic "Not for us, sorry." Which means that authors in the Romance field have more opportunities to start out on their own, then snag an agent afterward.

Having a contract in hand is not a guarantee. It does tend to make agents get all perked up, but they will still want to look at the book in question to see if what/how you write is a good fit for their list. So, going to publishers, landing a contract, then querying agents with that contract in hand may seem like "Oh, it was the contract that drew them in!" but you don't know if the author had just queried them as normal if the agent wouldn't have still been as interested. (Knowing that someone else is interested is sure to make anyone look at your work more closely, but if it's not well-written or not in a vein they like, they won't usually sign you just because you already managed to find an editor who wants the MS.)

Personally, I would have been very hesitant to sign with an agent who did not have advance-paying book deals under his belt. If it were a newer agent at a well-respected house... maybe. But someone who has been in the business for more than a year or two should be placing books with publishers their authors can't access on their own.

Just my $0.02. YMMV.

There might be benefits to having an agent, even with publishers who are open to unagented subs. For one thing, they may give more careful consideration to an agented submission, or it may land on the desk of the editor most likely to be receptive to it, and they might read and arrive at a decision more quickly, and of course, an agent can negotiate a better contract than the publisher's standard "boilerplate."

For instance, Tor and Orbit (two very big SFF imprints) do take unagented subs, but that doesn't mean you have the same chance of getting published by one of these without a good agent or that an agent getting you a deal with Tor or Orbit isn't something to be very excited about.

But if an agent has a ton of sales to presses that aren't known for advances/fair contracts or high sales or promoting their authors successfully, then that might very well be a warning sign. Some sales to these markets when they've also sold to better ones? Maybe not. It might just be a sign that the agent will keep going with a manuscript until they find something, even if it's not a market that's likely to make them very much money for the work they've done.

Whether it's better to trunk a book none of the bigger presses wants than to publish it somewhere, even if it's only with a press that will sell a few hundred copies of your book and not be findable on store shelves? That's a judgement call. But if an agent believes in a book and thinks it's good, they might be looking at the long-term career of their client. Maybe they think having the first book out there in the world where it has a chance of attracting some notice and good reviews, at least, will make it easier to sell the second one someplace better.

Disclaimer, I don't work in publishing, so this is based on my own choices about who to query, and maybe I'm completely wrong.

Well, as Roxxsmom pointed out, sometimes agented books wind up at smaller/ebook only/non-advance paying publishers because they just didn't sell to the bigger houses.

As you said, not every agent will have multiple big sales every year. (Although I would argue they are not necessarily as rare as you seem to think, but my genre is SF/F, not romance.) The important thing is deciding what will make you happy.

Personally, I want sales to an advance paying publisher. If that fails to happen, I'm more likely to want to self-publish than to approach the small pubs. (Been there with one already and they were super-nice, paid a small advance, but folded up shop less than a year after they contracted and published my novella.) So, when I was querying, I still queried widely, but I was looking for an agent who would start at the top and work down, rather than start in the middle and take whatever offer we got. Knowing what I was expecting from the process helped me focus on the agents who were most likely to help me achieve that.

Your minimum expectation should be finding an agent whose goals for your books and your career are in synch with yours. It's perfectly reasonable to just want to see your book in print and maybe make some money on the side. Or to only want a contract from one of the Big 5 (or their imprints). (Patience is key, though, if that is your goal.) Or to make money from your work, but taking it all from royalties because you write fast and frequently and need an agent who can keep up with you and keep putting your books in front of smaller, faster publishers.

Of course, an agent can't guarantee anything. But you need to know your own goals, and then try and find an agent who is genuine about meeting those goals. (Talking to some of their clients may help you figure out if the sales to e-book only pubs are due to not having contacts further up the chain or because they just haven't landed a big sale yet.) A red flag would be if someone is claiming to have a lot of industry contacts, but doesn't seem to have the track record to match.

The size of an agent's footprint means everything. You want an agent who has made several GOOD sales to TOP publishers in your genre. Otherwise, you're better off with no agent.