I wouldn't be surprised if they could. Probably not while it's in your pocket though. They're packed with sensors that can pick up a lot. Just putting a phone on a table with a keyboard on it is enough for it to be able read your keystrokes. They can certainly tell when you're doing something that should be raising your heart rate.

Cell phone services don't record your heart rate.

As has been noted on HN several times, there are cars that monitor when you have sex in them. You might be surprised what is known and by whom.