Find invalid data validation cells
<h2>Description:</h2>

<p>I have created a custom dropdown data validation list where I can choose among several values. These values on the dropdown list changes as I need (are defined in a worksheet column X).</p>

<h2>Problem:</h2>

<p>My problem occurs when I choose one of those values, let say Y, from the dropdown list and then I update the data validation by removing the last inserted value (deleted the Y value from column X). By doing this the value Y present in the worksheet is no longer valid so I would like to know if there is a way to obtain a list (array or string) of cells with the invalid data.</p>

<h2>What I have done/thought so far:</h2>

<p>I have searched in several sites and read similar questions but I cannot find anything usefull. I thought about looping all the cells and check if the value is valid but since I have a huge amount of data I think that it is not the best approach. </p>

<p>Since Excel already mark these invalid data with a red circle maybe it could be possible to get the address of those marked cells?
Thanks in advance! </p>