If you know for sure, there’s a 4th ball, find it. Take a break, and come back to look, fresh set of eyes does wonders.

Always find the missing piece you drop, so it doesn’t cause potential damage and/or shorting. Did you here it drop into the cabinet, or is it still on the playfield?

Countless times I couldn’t find it in the first 10 minutes. Then I come back, and poof, easy find.

Bottom of cabinet.

This.

I've had screws, spacers and other metallic items hide all over the cabinet, look carefully with a flashlight. Sometimes they get stuck on the speaker magnet (or even inside the speaker) but that shouldn't happen to your ball.

I have had a number of pins I purchased, played to make sure they were worked, removed the correct number of balls, transported and setup and later found random balls hiding in the lower cabinet. Still surprises me to this day how they didn't make a racket when packing/unpacking so I make a point after setup to go looking for spare balls, nuts, washers, machine screws, etc that have accumulated down there.

If you have kids, bribe them with $5 to find it. Maybe drag a telescoping magnet through the wire loom.

If you've just been looking only from one side of the cabinet, move to the other side and change your perspective. Sometimes an object you're looking for can be spotted from another direction.

It's not here is it?

https://pinside.com/pinball/forum/topic/tech-monopoly-stuck-ball#post-1329262
monopoly-ball-stuck (resized).jpeg

You could also check the ball jail / lock mechanism maybe. Seems as though sometimes balls can go in there and not be ejected or recognized upon starting another game.

See here:

https://pinside.com/pinball/forum/topic/monopoly-ball-jail-ball-lock-issues

Do what I (and apparently someone else) did: Just add another ball. The extra ball will eventually resurface. I found TWO extra balls inside my Halloween - both showed up (on separate days) on the playfield. To this day it still doesn't make sense.