Jane says "I could have pointed out the door" firstly because she didn't actually point it out, but secondly because at the time, she didn't even think of so doing. I'm not reading the passage to check, but quite possibly she was on her own. Why would she point out the door? Who to? (I know - the reader).
Using conditional perfect emphasises the fact that it didn't happen. She's only telling us she could have, so we know the source of the laugh was close, and Jane knew exactly which room it was coming from.
If she'd said "I could point out the door", that wouldn't really be right. I'm sure in common parlance some people would use this form in such contexts, but it's a bit sloppy. She could point out the door right now, or tomorrow. In fact, sometimes we say I could [do X] when we mean I was able to, and in fact I did [X]. It's very loose phrasing that doesn't really specify when you could have done it - or even if you did it.