I (32F) caught my bf (30M) with his ex after he told me he was somewhere else. Am I ignoring red flags?

I (32F) have been with my bf (30M) for a year, and we live about 2 hours apart.

A few days ago, I asked him what he was doing. He told me he was out taking pictures and was going to eat at McDonald’s. I happened to be near his town that day, so I stopped by without telling him. That’s when I found him with his ex and another couple. He wasn’t even in his own car. His explanation was that a friend had dropped him off there and then left because of an emergency, and that seeing his ex was just a coincidence. However, he never mentioned any of that to me beforehand. They both were far away from their town, like an hour from their town.

Looking back, I’ve noticed few times when they seemed to be in the same place at the same time based on IG stories. I didn’t think much of it because they’re from the same small town and share a lot of mutual friends, but now I’m questioning everything. For context, they dated for 4 years, and he has a history of getting back together with her while in other relationships.

I broke up with him after seeing them together. Now he’s begging for another chance, saying it wasn’t what it looked like, that he has no feelings for her, and that they’re just friends. He even sent me screenshots of messages with his ex where he was talking about how much he cares about me.

How can I tell whether his explanation is genuine or if I’m ignoring red flags? Has anyone been in a similar situation?