I posted on the issue tracker and a few other places about this and was shut down. Apparently it's a support issue, not a technical one. From my (limited and po

I posted on the issue tracker and a few other places about this and was shut down. Apparently it's a support issue, not a technical one. From my (limited and potentially erroneous) understanding, Signal hashes each of your local contacts and uses that hash to query whether the number is registered with OWS. If so, you receive a notification to that effect. Essentially, you can identify Signal users without actually communicating with them. Though the same is true with PGP if you use a public keyserver. That said, it gave some friends quite a fright. It's really unfortunate this wasn't opt-in or publicized before-hand.