For anybody whom might have any ideas about the behavioral of cats. I own 2 cats and...?

I own 2 cats and found that they "developed" a mob of other stray cats or perhaps they are owned...i was sleeping soundly last night and woke up at night to hear 2 or more cats around my house making odd meowing noises for an elongated amount of time. I noticed that the meowing would settle or lessen and then get louder again and more noticeable. It was almost as if that one of the cats was sounding like it was being hurt but i know the cats were not fighting. It was almost creepy and frightening. The best guess that i had in my mind was that they were mourning over a death of another cat...but i wanted to ask you...what could it of possibly ment/been?