Like I said above, it's not a STEMI, I described my reasons why above (
https://www.emtlife.com/threads/stemi-mimic.48053/#post-681280). That's OK. Nobody is going to knock you for doing what you think was right and trying to follow your protocols.
There is the patient's baseline and then your baseline that you took. If you do serial 12-leads, you can see if the next 12-lead deviates from the baseline set that YOU took.
When you put the patient on the monitor, either a 3-lead or 4-lead, it is usually called "monitor mode". When you do a 12-lead, it is usually called "diagnostic mode". They use different filters that can hide, create, or reveal ST changes. It is possible to put the patient on the 3 or 4 lead, see ST changes, but for those ST changes to go away when you do a 12-lead. At the same time, it is possibly to put the patient on the 3 or 4 lead, not see ST changes, but for ST changes to be revealed when you do a 12-lead. Usually though, if there is ST elevation or lack of, it'll be the same for the 12-lead. It's possible for there to be a difference in diagnostic and monitor mode, something important to note, but it is not usually the case. Christopher Watford wrote all about it here
http://ems12lead.com/2014/03/10/understanding-ecg-filtering/, but most of it goes above my head.