When Obi-Wan and Yoda return to the Jedi Temple, they discover that many Jedi knights, padawans and younglings were killed. Obi-Wan records a warning message for all Jedi to stay away from the temple.
He then goes through the security recordings to discover why some padawans and younglings were killed with a lightsaber blade.
Here is the script for that scene:
185 INT. CORUSCANT-JEDI TEMPLE-CONTROL CENTER-NIGHT
OBI-WAN enters the Main Control Center with YODA and heads for the hologram area.
OBI-WAN: Wait, Master. There is something I must know . . .
YODA: If into the security recordings you go, only pain will you find.
OBI-WAN: I must know the truth, Master.
OBI-WAN moves to a panel and flips some switches. He sees a HOLOGRAM of ANAKIN slaughtering JEDI, including the YOUNG ONES. OBI-WAN and YODA react.
OBI-WAN: (continuing) It can't be . . . It can't be . . .
As ANAKIN surveys the carnage, a DARK-ROBED SITH LORD enters. ANAKIN turns to DARTH SIDIOUS and kneels before him.
ANAKIN: The traitors have been taken care of, Lord Sidious.
DARTH SlDIOUS: Good . . . good . . . You have done well, my new apprentice. Do you feel your power growing?
ANAKIN: Yes, My Master.
DARTH SlDIOUS: Now, Lord Vader, now go and bring peace to the Empire.
OBI-WAN watches in horror. Tears well up in his eyes.
As shown in this clip from Rebels, the message did not mention that Anakin had become Darth Vader. This knowledge would have been very important to the exiled Jedi so as not to trust Anakin, yet he does not share it.
You could say that Obi-Wan learned shortly after he made the warning message that Anakin betrayed the Jedi, and not before. Even so, he could have altered the message.
Why did he not warn the surviving Jedi about Darth Vader?