Region3
Ryder Cup Winner
A situation cropped up today (not in my group) and I was asked afterwards if I knew the answer. I said "no, but I will find out for you" 
Player hits his tee shot into a tree near OOB so plays a provisional ball.
They find his ball, but the OOB markers aren't particularly well placed and determining where the OOB starts can be difficult. All 3 of them decide that the ball is in play, and the player hits his ball back out onto the fairway.
Another group coming back the other way were watching, and as they passed each other they said that they thought the ball was OOB. Another discussion ensued and all 6 of them agreed that the ball was indeed OOB.
What is the penalty and relevant rule (if any) for the player playing a ball that was not in play (OOB) albeit in good faith that they thought it was in play?
My best guess was that it would be a 2 stroke penalty for playing a practice stroke, but it is purely a guess.
Can anyone confirm or put me right please?

Player hits his tee shot into a tree near OOB so plays a provisional ball.
They find his ball, but the OOB markers aren't particularly well placed and determining where the OOB starts can be difficult. All 3 of them decide that the ball is in play, and the player hits his ball back out onto the fairway.
Another group coming back the other way were watching, and as they passed each other they said that they thought the ball was OOB. Another discussion ensued and all 6 of them agreed that the ball was indeed OOB.
What is the penalty and relevant rule (if any) for the player playing a ball that was not in play (OOB) albeit in good faith that they thought it was in play?
My best guess was that it would be a 2 stroke penalty for playing a practice stroke, but it is purely a guess.
Can anyone confirm or put me right please?