You can check your activity online to see if maybe someone did deposit money into your account...maybe your parents? But that would be the easiest way to find out why there is random money in your account. Another thing is I know some savings account do this but i don't think checking does which is interest fee? Some are annually or quarterly.
You should contact your bank anytime you have extra money in your account that you did not deposit.
Knowing it's not yours and taking it is still considered theft.
what do the details in your acct say - do just go by an ATM machine balance - look at the transactions in your bank acct online
call them and ask what's going on