cat meowing

I have 2 cats that are both 13 years old.

Only one of them has started the oddest behavior over the last year. She will take one of her fake mouse (she likes the bright colored:red, orange) and will walk down the driveway with it in her mouth and have the oddest, loudest, most agonizing meow. Sounds like she is upset or in heat.
She then drops the mouse and leaves it. I will pick it up, put it on the table outside and she will come back later and repeat the entire sequence.

Does anyone know what that means?

I think it's something similar to catching their prey?!.

Rosie does this nearly every night with a small catnip sack. When i first heard her coming down the stairs making those noises you described i've never shot out of my seat so fast thinking their was something wrong. But now as soon as she gets to the bottom of the stairs she drops it but carries on making the yelling noises until i go and see her and tell her what a good girl she is for mummy, then i pick the toy up and she trots off quite happily

She's just presenting you with a gift. The noise is her way of announcing her accomplishment in the hunt.