Let me know if this is a duplicate. The answers I found focused more on agency recruiters, so there isn't a double-application issue here, and they have an incentive to withhold information if it helps placing you; I have a different concern.
I'm actively interviewing and had two on-sites with high-profile tech companies. Waiting to hear back, but things went at least ok. Not wanting to let the pipeline drain and curious about other opportunities, I reached out to a friend at a similar company for a referral. An in-house recruiter followed up. In an email, they asked if I'm interviewing anywhere else and how far along.
My concern is that one or both of the places I've had on-sites say "no," and that makes this company think that I might not be a great candidate. Or they say "yes," and if I tell the recruiter "no" right now, I'll have a follow up of "actually, I have an offer from X; can we speed up the process?"
I already made the mistake of telling the first two recruiters I'm interviewing at the other (luckily they're both at the same stage), so at this point, I'm hesitant to give the new recruiter more information than is needed.