I can't tell you how many times I've inadvertently blown a secret just because I didn't know it was a secret. It's to the point where as soon as anyone asks me, "Hey, can I tell you something?" I ask for these details.
People think other people should always be able to tell. Some of us can't. I don't just not read minds. I also don't (usually) read social cues.
There's definitely an audience for this. It just isn't you!
If someone tells me something in a one on one or small group conversation, my default assumption is that I should not share that with anyone else. The idea that I would have to tell someone to not share details of a private conversation is crazy to me. Unless stated otherwise, respect people's privacy.
Obviously there are people that gossip, which is kind of the same thing, but also is very much frowned upon.
For example, once a friend told me they were going on vacation to a particular city, but didn't mention it was for another friend's anniversary surprise. When the other friend mentioned they had a trip planned to the same place around the same time, "Oh! [First friend] just told me she's going to be there too! Maybe you guys can meet up together or something! That'd be so fun!" was out of my mouth almost immediately. It never even entered into my mind that there was a reason not to say anything.
I inadvertently ruined a fun surprise just because nobody told me it was a secret. If they had, I wouldn't have said anything!