When I got my first run Spidey from the original home owner, it only
had 3 balls. Because it had prehistoric software, the game (and the
owner) didn't notice a ball was missing. When I briefly test played it
(it was minty) at his house, everything seemed normal. Got it home and
noticed one missing. It was way up in the top right corner of the
playfield hidden under the right ramp return. If you saw it there, you
would say it's impossible to get a ball stuck there. Welp, somebody
did. Think I fished it out with a magnet tool.
You'll find it. They find ways to get into places they really
shouldn't be. It may or may not require tools to extract said ball,
but you'll find it.
-phish