Sentinel is an Android-based application developed by Samourai for tracking the balance and transactions of your offline and cold-storage bitcoin wallets. The program exclusively uses public keys to ensure that your private keys remain offline and secure.

With Sentinel, you can track any type of bitcoin address or extended public key, including those created by Mycelium Wallet, Coinomi, Ledger, Trezor, and BRD. The program fully supports SegWit, allowing you to easily track your SegWit addresses.

One of the standout features of Sentinel is remote deposit, which enables you to receive bitcoin payments into any tracked wallet or address without needing access to your private keys. Additionally, you can broadcast signed transactions from your offline wallet to the bitcoin network when you need to send bitcoin.

Sentinel also offers currency conversions for USD, GBP, EUR, CNY, and RUB. You can even add an optional PIN code with PIN Scramble technology to help thwart keylogger attacks.

As fully open source software, Sentinel's code is available to review publicly on Github. Overall, Sentinel is a reliable, user-friendly tool for securely tracking your bitcoin transactions.