There's a heart right as Section L turns into Section M:
You'll notice the bomb explodes a bit over the ground, signifying that there's indeed something hidden. I didn't even notice it as I was playing, it was only later when I was watching the vid I had captured when I caught it. I tried getting it on a later run to see how many points it would give, but try as I might, I just couldn't get the heart to appear again. I continued over and over again (the checkpoint is right before this spot) but to no avail.
Unless credit-feeding deactivates these hidden bonuses, I have no idea what's going on there. Maybe the bonus appears only if you start from an earlier checkpoint or something. In any case, it's going to take two or three speed-ups to shoot that heart, there's not much time after bombing the spot before the heart scrolls off-screen.
I also made
a short vid showing to get through the pinball section in case anybody's interested - there's nothing fancy about it, just get through as fast as you can.