Plastic ball in water pipes?

So I go to replace a shower cartridge today after I did some work yesterday where I shut off the main water valve in the house and later that night they had no water in the shower upstairs. I replace the cartridge and from no water we now have cold but no hot water. I flushed out the lines before putting the new cartridge in. I end up seeing something at the valve inlet on the hot side and try to get it out but no luck. I cut the copper line going up to the valve and now at the top of the pipe is this little white ball floating in the pipe. Just a little bit smaller than the pipe. Since it was floating I guess it is plastic. Does any of you know what the heck it is and how it might have gotten in the waterlines?

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