How to address concerns about my best friend's boyfriend possibly hitting on me?

The last couple of weeks, my best friend’s boyfriend has said and done things which are out of the ordinary in context to our relationship – but nothing overtly inappropriate. As a matter of fact, one of them may be seen as a compliment, and the other a simple friendly gesture.

However, in the past he hasn’t said or done anything similar to either of the things he said/did within the last two weeks. On top of that, he’s been opening up to me about the issues he’s having in his relationship with my best friend. My best friend has also been telling me about issues in their relationship; specifically, certain things he’s said to her, which make me wonder if there is more to what he said and did.

My dilemma is this: Because I’m unsure about what exactly he’s doing and if there really was nothing to these things, do I talk to my best friend about it? I’ve been friends with her for just over 15 years now, they have a baby together, and they’re having issues lately. I don’t want to make a mountain out of a molehill.

If it helps at all, about 3 or 4 years ago, my best friend approached me and asked if I would have a threesome with her and him. It was originally his idea. Edit: The threesome never happened.

Because of all the factors – I don’t know what to do and I’m confused about his behavior in general. I don’t tend to ask these questions as I know a lot of people don’t like answering them. But I think I genuinely need outside perspective for this one, and whatever help I get will be very appreciated.