It's in the VerifyUserHasTeam middleware. See https://github.com/laravel/spark-aurelius/blob/9.0/src/Http/Middleware/VerifyUserHasTeam.php
If Spark::usesTeams and the user doesn't have any teams, it'll redirect the user to /missing. You can try overriding or creating your own middleware that get's used in this instance.
It's being used in the install stubs that get copied to app/Http/Kernel.php .. see https://github.com/laravel/spark-aurelius/blob/2b4a4225c2045c27597c807f3e786803bed7b53a/install-stubs/app/Http/Kernel.php