A wee bit of unsolicited advice for your systems administrator -- assuming you have one. No need to reply in this forum, this is all just food for thought. Not finding root cause here can lead to a repeat performance later on.
-- check with anyone who has systems admin access (or equivalent) to your site and your servers: has anyone been phished?
-- where are these servers (I assume more than one) physically located? Are they in someone's garage, or do you work through a service provider? If the former, that's not really ideal, to put it mildly. If the latter, I hope that someone with authority (or, at least, someone who sounds like they have authority) from steamup.com needs to get on the phone with that provider and tell them you're taking your business elsewhere unless they get to the bottom of this.
-- change access passwords; use an online password checker to ensure that no-one's choosing a password that's easy to hack
Hopefully, if you're already working with a professional or have taken the steps (plus a few others) note here. Let's hope this is a one-off.