When my parents started losing things (wallets, keys, TV remotes) I started relying on similar little tracking fobs. They had a separate transmitter instead of using a smartphone bluetooth but the problems are probably similar. Every model we could find had very low volume when they beeped - made it hard to find them even under good conditions. The other problem we had was short battery life on the individual fobs and that most of them wouldn't reset when it came time to change the batteries. We tried several kinds from Brookstone, an electronics store, and stores in China through Aliexpress.com. Bottom line was that I'd say they were helpful IN the house for finding things behind a chair or under the bed but not much good outside the house and the battery issue was why we finally abandoned them all.
A suggestion - got to walmart.com and search for "key locator" if you don't have a bluetooth smartphone or try "tracking device" if you do. They have quite a few examples.