Idk, I've made direct eye contact with an old classmate years after graduation and he absolutely recognized me and saw that I recognized him. We did a short nod of acknowledgement and never spoke a word. Haven't seen him since.
Just because you're classmates doesn't mean you need to be apart of their lives or interact with them after school, or vise versa.
Try not to think about it, but if you're worried it couldn't hurt to message them back and say something "Hey I saw you at the thing but you looked a little preoccupied and I wasn't feeling up to interjecting. How have you been, did you enjoy your time there?"
Acknowledge the thing you're anxious about and use that to branch into a common topic, the event you were just at. You could use it to ask why they were there which could lead to talking about family, what they would have rather been doing which goes into hobbies.
Just talk to them if you're worried about it, but honestly you don't need to worry about it. That's just life sometimes. We go our different ways.