Is it possible to have had a heart attack without any symptoms?

Yes: It is called a silent heart attack. Usually there are some innocuous symptoms but there could be none. The only way to know for sure is to have it evaluated by a physician.

Yes: Or at least symptoms that are mild enough to be ignored. We can see evidence of prior heart attacks without the patient having any knowledge of having one. Some diseases like diabetes blunt symptoms and some people have atypical symptoms that mimic acid reflux or other conditions.