Visible crew/equipment: During The Music of the Night, you can see a camera shadow pass over the Phantom's and Christine's face. It's around the part where he says, 'Let your fantasies unwind'. (00:38:00)
Visible crew/equipment: As Raoul's car is leaving the Opera House, a crewmember is reflected in its windshield. (00:48:05)
Visible crew/equipment: Just before Raoul jumps down the hole after the Masquerade, Patrick Wilson is obviously replaced by a stuntman who's not even trying to hide his face. He is the same stuntman used for the Phantom in the graveyard swordfight. (01:25:35)
Visible crew/equipment: During the graveyard swordfight, the Phantom's stuntman sometimes visibly replaces Gerard Butler. He has thin and very arched eyebrows and the mask doesn't fit properly around the nose. He is the same stuntman used when Raoul jumps down the hole after the Masquerade. (01:40:45)
Visible crew/equipment: During the second kiss, the shadow of a camera is seen on Christine and the Phantom. It's especially obvious behind his neck. (02:06:05)
Visible crew/equipment: When Christine and Raoul are riding the horse out of the graveyard, it is obvious that Emmy Rossum has been replaced by a stunt double.
Visible crew/equipment: Right after Christine is shown on stage singing "Think of Me", the camera pans out to show the audience members. When it gets to the back of the room on the floor, you can see at least 5 dummies used as seat fillers. They look to be only torsos and heads sleeveless t-shirts on.
Visible crew/equipment: When Raoul jumps down the hole into the mirror room, you see he is alone. Between when he jumps down, and the appearance of the phantom's reflection, if you slow down the movie you see a man in a gray shirt in the mirrors behind Raoul. He appears in at least three mirrors. Slow-mo is not essential but does help.
Answer: I like to think that he would always place a rose there, possibly every week or so, and they never had the ring on them. And he knew When Raoul would be there, so he placed them when he knew he would be alone with her. As for the two years later, since the phantom "knew" when Raoul came... he especially put that ring there. Just to let Raoul know about his love for Christine and the fact he is still living. He knew Raoul would visit then, and knew what to do. (This is only a theory I have proposed. As for a genuine answer... I am not aware of one.) Hope this maybe helped.