If it hadn't been a dramatic musical discovery moment, Spike would probably have fallen out of the chair in shock and dismay. Or, at least, had a few minutes to spare for swearing. Of course something had arranged things so Buffy heard every word. Stupid to think otherwise. No point in a musical acknowledgement of the things you're not saying without the person you're not saying them to hearing the entire sodding business.
All that aside, he might have fallen out of the chair in shock and dismay just because her response was Coldplay. Someone clearly hated them.
Spike stood up and walked over to the door, annoyed. Mostly with the situation, but sure, a bit with Buffy. Which part of Hello, I'm an evil monster who you're supposed to slay, and I'm not going to let you feel sorry for me and think it's affection after all the shit I've done to you did she keep not getting? Not that they'd had it out, not in so many words. Which was probably why they were now singing. Which he was not going to keep doing. No, he was going to keep his mouth shut, show the lady out the door, and then go get thoroughly drunk.
Which is exactly what didn't happen, of course.
"Your sweet whisper I just can't handle Take your hair in the moonlight Your blue eyes -- goodbye, goodnight I should tell you I should tell you I should tell you I should -- no! Another time -- another place Our temperature would climb There'd be a long embrace We'd do another dance It'd be another play Looking for romance? Come back another day"