Unexplained deposits could be trial deposits from attempting to link your bank account with another account. If you were to link a BoA account to a Chase account, BoA would make a couple deposits (usually less than a dollar) into the Chase account. These would show up as "BANK OF AMERICA TRIAL DEP", or something similar. In order convince BoA that your really own the Chase account, you need to confirm the value of those deposits.
So what should you do if you suspect criminals have your account number? Change the passwords for your bank account and your email account and notify your bank immediately. If someone can link to your account they will be able to draw funds from your account. If someone got your account number they can try to link it to your bank account. Then they need to log into your account to find the value of the trial deposits, which they can do if they have your online banking password. If they have access to your email account, they can use it to reset your online banking password. If there's any chance that your password was compromised, or your email password was compromised, change them. Change them even if you think there's no chance they were compromised. If you used the same password for other banks or websites, change those too.
It's unlikely this is a trial deposit because there's only one, and it's so large. But if you think someone is up to something suspicious with your accounts, changing your passwords is cheap insurance. That should be the first step if something suspicious like this happens.