My cat went missing friday night, and has been found dead?

I have 2 cats, bruno n lola. i let them out friday ttime when i came home from work as i always did. I always leave the door open so they can come in and out. The last time i saw her was around 6.30. At 9.30 i called them in but ther was no sign of lola. i stayed up hours waiting for her but no luck. I was worried straight away as i never leave my cats out alone and they never go far, ther always in and out. I went out to find her but no joy. She still wasnt home saturday AM and by this time i was extremely worried. i put posters up hoping someone would see her. Sunday came and still nothing, until lunchtime a neighbour said she was behind her wheelie bin dead. And she was, no blood or injuries. Could she have been poisened? She was only 15 months and was my little baby.. i miss her soo much, i just hope she didint die in pain. Bruno is lost aswell.. totally devastated