Although ST-depression in leads V1-V3 may represent anteroseptal ischemia, it is important to consider other causes such as isolated posterior STEMI which is the most common type of missed STEMI. Consider the following differentials:
Posterior STEMI (mirror image of septal STEMI in leads V1-V3)
Large R waves (instead of Q waves)
ST-depression (instead of STE)
Upright T waves in V1-V3 (instead of inversions)
Anteroseptal ischemia
Miscellaneous
RBBB
Hypokalemia, etc.
Check out the following link to learn more about Posterior STEMI