Who Was Buried in the Grave and Where Did the Body Go?

This is a mystery story. I am hoping my wonderful readers will have some ideas for solving this bewildering mystery.

THE BACK STORY

My daughter-in-law was going through her deceased grandmother's papers recently and came across the photo you see on the left.

Robert Leslie was her grandfather, and this is his grave. Robert Leslie is buried in Pine Hill Cemetery in Scarborough Ontario. However the family had no clue who was buried with him! They did not know of any Christine Leslie and Robert's wife Bernice (my daughter-in-law's grandmother) just died this year (2015). 

The plot in the cemetery had been purchased for two - Robert and Bernice. The stone had space to the right of Robert's marker which was intended for Bernice's name. 

THE MYSTERY

Robert's daughter does not know why a marker with the name Christine and the dates 1922-1999 were on her father's grave. As far as the family knew, Robert had no sisters. So my daughter-in-law asked me to investigate.

Due to Canadian privacy laws the last publicly available census for Ontario is 1921 so I could not look for Robert and his family to verify there was no sister Christine. Birth records are also not available for the time period I needed. So I contacted the cemetery. 

They informed me that the only person buried in the grave was Robert Leslie and there was only one marker on the stone. They also noted that the right side of the stone was left empty and a notation on the records stated  a marker with Bernice's name on it was to be added on her death.

I sent the cemetery the photo we have and asked if someone could actually go to the grave and verify that there was only one marker on it. They very kindly agreed to do so.

The photo above is the photo they sent me on October 1st this year. As you can see it is the same stone but it only has one marker - for Robert Leslie. The marker for Christine is gone.

I asked the cemetery if they would check their records for a Christine Leslie born 1922 and died in 1999 buried elsewhere in the Cemetery. A search was made with zero results. There was no one named Christine Leslie buried in their Cemetery and they had even searched their records by first name (Christine) and surname Leslie to see if anyone was buried there in 1999. The search came up empty. No Christines and no Leslies were buried there in that year.

We have enlarged the photos to verify that they have not been altered. The photos are real. At some point in time the photograph of Robert and Christine was taken. We do not know when but we know it was Robert's son who took the photo and gave it to his mother. 

QUESTIONS

So many questions come to mind: Who was Christine? Why was her marker put on Robert's grave and why was it removed. Who put it on and who removed it? Was she ever buried in Robert's grave or was it just her marker on the stone? 

Did the cemetery make a mistake and put the marker for Christine on the wrong stone and they removed it? But if that was the case the office would have found a burial record for a Christine (surname unknown) buried there in 1999 - and they found no such record!

Did someone sneak into the cemetery at night and bury Christine's ashes in Robert's plot, then place a marker there? If so, who removed it, and why?

THE CHALLENGE

My challenge to my reader is: Put on your sleuthing caps and see if you can solve the mystery of Christine! If you have access to the Toronto newspapers for 1999, maybe you can find mention of a Christine dying that year.