I think it is unlikely that you will find a device that can hide with the bike and works well; however, if as a touring cyclist you are concerned about the complete set-up consider a disposable phone that offers gps tracking from its service provider. It would at least provide a window of opportunity to track a thief by just leaving it buried within the panniers. Provide the smallest cost for a useful level of effectiveness, and also serve as a backup to your primary phone should something happen to it (multi-tasking is a wonderful thing on bike tours)...