Lucy the Ghost Cat is one possible reward from Shadowlands trainee callings quests, depending on which trainee you get. Once every five minutes, after combat, your trainee may give an emote that they found something on the ground, and a sparkling pile will appear for you to loot. If you aren't paying attention, you may miss it. This addon will play a sound when that happens, so you don't miss it. One of the items you can get is Lucy's Lost Collar, but you can also get a lot of other nice things such as portal stones, pet training stones, etc.

Each covenant has one specific trainee that can find things on the ground as shown below:

Kyrian: Deos
Necrolords: Dregs
Night Fae: Twinklewings
Venthyr: Drippy

See the WoWHead link below for full details of other items that might drop.

https://www.wowhead.com/news=320310.2/lucy-the-ghost-cat-and-other-rewards-from-shadowlands-trainee-callings-quests

Updated version 1.1 adds two slash commands:

/lucywatchsound

cycles through four different sound options

/lucywatchtimer

toggles a feature to show a 5-minute timer after an item is found

Old version:

If you do not like the default sound, you can change it to one you prefer by opening LucyWatch.lua and changing the number on the fourth line down as seen below:

-- Set the default sound to play
-- 37666 is the Boss Warning sound. If you don't like that, then change the number in the line below to a sound you prefer.
-- Examples: 888 = Level Up sound, 8959 = Raid Warning sound, 8960 = Ready Check sound, 37666 = Boss Warning sound
local theSound = 37666

This WoWHead page has a lot of other ideas for sounds to use:

https://www.wowhead.com/sounds/user-interface