Yeah, conversations are fun.
Yes, there would always be slayers who were killed but unknown vampires, you are right. Angelus could have been one of them. There is certainly no reason why you can't have Angelus with a slayer on his record, although I think if you wrote it, you'd need a line to explain why that was an unknown fact.

I'm leaning towards what was said on the show and Angelus saying he wouldn't go looking for slayers but he wouldn't run from them, either.
Sorry, I think I gave a false impression there. He didn't actually say that, so clearly. That was just the implication that I took from his tone.
The actual exchange I was thinking of went like this:
Angelus: You've got me and my women hiding in the luxury of a mine shaft, all because William the Bloody likes the attention. This is not a reputation we need.
Spike: Oh, I'm sorry. Did I sully our good name? We're vampires.
Angelus: All the more reason to use a certain amount of finesse.
Angelus: No. A real kill. A good kill. It takes pure artistry. Without that, we're just animals.
Spike: Poofter!
Angelus: You can't keep this up forever. If I can't teach you, maybe someday an angry crowd will. That... or the Slayer.
Spike: What's a Slayer?

And we are certainly not in disagreement. Maybe in debate? *g* Thank you for the conversation.