No notification... they never want you to know that buses are out earlier than the 45min - 1hr before park opening that CMs are trained to tell you. If you ask, they will consistently tell you that.
the 2:30 HS bus on Thurs was per a guest who claims to have been on it. The 4:30 am let in of the line at the taps on Thurs was per a CM at GE and I did talk to a few who was said they were in at 5:15. Talked to some who Lyfted at 5am who assumed the buses weren't out but also heard about the super early bus rumor afterwards. We got to taps around 6:30 after bag check (picked up 6:10 ish) and it looked like people have been going in for a long time with the line for Smuggler's being wrapped around for a while... plus we were already down to boarding group 40 at 6:30 when park didn't open until 8.
We heard the buses were out much later for Fri 9am opening. We again got there at 6:30 at taps and took boarding group 24 and 25 (we had an error for 1 in my group trying to check in immediately after taps and it took a while to resolve).
I typically went with the earlier of 630am bus (trying to get there 10 min before that) or 1 hr before earliest park open and found that, at least this past week, being special circumstances with the grand opening paired with Thanksgiving, buses were even earlier. Thanksgiving Day ,for instance, we tried for 6:30 bus for AK 9am open, and the bus driver said that he had been doing runs since 5am.