The metadata_exists() function should return the correct value if the get_$meta_type_metadata filter returns a non null value

[xknown]

The line 324 should return $check instead of true, otherwise the filter doesn't normally work when someone wants to always return false for a given meta key.

Here's a quick way to reproduce it

> add_filter('get_user_metadata', '__return_false'); return metadata_exists('user', 1, 1)
TRUE

Return $check instead of true.