Magicfly Wireless RF Item Locator Key Finder 1 RF Transmitter and 6 Receivers
Bought these to use on my tortoises and they work like a charm.  They are loud enough for us to hear, and if they happen to knock them off we are able to locate them in the bushes.  They seem to hold up well in the rain.  I haven't used them in side so not sure if they will go through walls.  These work great for the purpose we purchased them for.