I"m sorry that you can't find your frog. Are you sure she just isn't hiding in the tank? (guess you would be)

Water bowls are the best way to go, but all over the house. You never know where these guys will turn up.

On another note, did you say you placed the new frog in the tank with the old frogs within a day of purchasing it? You should always have a quarantine period just in case the new frog has something that could harm your old frog. She probably should have had her own tank for a good few weeks before throwing her in with your males. It's always risky adding new frogs right away.
-----
I think my frog owns a megaphone....

White's tree frogs : 1:1:0
Mantella viridis : 1:1:2