Instead of just trusting the web based interface from the company that tells you all about your transactions in your savings account, your interest earned, and all that, open that same deposit address as a watch-only wallet to really see what's going on.

You can do this easily with Blue Wallet. Install it on your phone, open it, and add a wallet by clicking on Add now. Click on Import Wallet at the bottom, and then simply paste your deposit address there. Now just wait for a few minutes until it finishes the import process.

Once it finishes, have a look at the transactions. Compare them with the transactions the company website shows you. Remember, don't trust, verify.

First, check the incoming transactions. Make sure that every deposit you've made shows up correctly in the watch-only wallet. Then, make sure that any interest paid from the company is actually deposited into your account. The interest paid to you will probably not show up as a real transaction, but your deposits will.

Second, check the outgoing transactions. If you've withdrawn at any time, check if you can find these transactions. You probably won't be able to find those transactions, because your withdrawals will come from another address. Then, check if there are other withdrawals that you haven't made. You probably will find withdrawals that you haven't done. That probably means that your bitcoins is being used to pay for other user's withdrawals.

If you do this you'll see exactly what's going on with your account. You don't want to be in a situation where everyone withdraws their Bitcoin at the same time and you end up with no Bitcoins at all.

This is obviously how it works, and probably most people know this, but I think seeing it for yourself makes it more painfully obvious and encourages people to hold bitcoin in their own cold storage instead.