offsetExists($index) examines the cache, not the inner or outer iterator.

<?php
        $cache = new \CachingIterator(
            new \ArrayIterator(['a', 'b', 'c', 'd']),
            \CachingIterator::FULL_CACHE);

        $shortRange = range(0, 1);
        $fullRange = range(0, 3);

        foreach ($shortRange as $index) {
            $cache->next();
        }

        echo PHP_EOL . 'The cache' . PHP_EOL;
        var_export($cache->getCache());
        echo PHP_EOL;

        foreach ($fullRange as $offset) {
            print_r("cache offset '$offset' " .
                ($cache->offsetExists("$offset") == 1
                    ? 'exists'
                    : "doesn't exist"
                ) . PHP_EOL);
        }
?>

The cache
array (
  0 => 'a',
  1 => 'b',
)
cache offset '0' exists
cache offset '1' exists
cache offset '2' doesn't exist
cache offset '3' doesn't exist