What does it mean when he sees me then leaves?

I (32F) was in a situationship with a guy (32M) on/off since April. We had a 6 week-gap then saw each other again back in September.

Last time we saw each other, we'd had a nice night, then went to work next day. We'd texted and then from there left it at that.

I've not seen him in weeks, figuring that if he wanted to, he would make the effort, but no negative conversation.

I was at the pub the other night with my brother (whom he doesn't know), in the smoking area outside. The guy comes out and immediately sees me, then turns around. I was clearly the reason he'd immediately turned around, but why? When he's seen me out before, we've chatted. This time was completely different. What's happened?
What does it mean when he sees me then leaves?
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