I guess there are a few possibilities.
1) SVPM found evidence in the trash? Bus Ticket, something?
2) Luka talked.
3) Possible Accomplice spoke up?
Pure speculation...
4) The park is also used by gays for cruising. It could be someone who noticed something strange but didnt dare to tell till now. Fear of being outed. A lot of 'straight' men with wife and kids use area's like that to have anonymous gay sex. Just speculation...
http://www.cruisinggays.com/montreal/areas/4528-parc-angrignon/#