I appreciate both responses. Thank you guys. I will check these things out. I am particularly suspicious of the radiator cap, since I think it is an older one, and I noticed some small white crystals left over from evaporated coolant.
I wouldn't rule the expansion tank out entirely - when mine went it didn't dump until I had shut the car down. Didn't see/hear anything until I turned it off in my garage and heard the car start peeing. No drips on my driveway or street.
Just something to keep in the back of your mind.