flat spot on replacement tire- 2x?

@ 36K miles, I got both front tires replaced (the tire place said left front had 3 holes, so could not repair; not sure if I believe that). Drove cpl days, felt thumping. They test drove it, asked me bout it, test drive again. They said the tire mustve been sitting in warehouse for months & got a flat spot. They replaced; no charge (I did purchase hazard protection). Now, weeks later, Im thinking, maybe, another flat spot. But I can barely feel/hear anything; its very faint, if theres anything at all. Should I take it back? If I dont, whats the worst that can happen? Will tire suddenly blow up, or will slowly get worse?