Normally if you open an SVG file with an image xlink:href pointing to a ai/pdf file it links and shows the file where the image is. However, if that image is inside a symbol (I found some issues with <defs> too) it does not link and is missing entirely.

Broken:

<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 400 400">
    <use xlink:href="#symbol" width="400" height="400" />
    <symbol id="symbol" viewBox="0 0 400 400" >     
        <image width="400" height="400" xlink:href="linked.pdf" opacity=".4" />
        <image width="400" height="400" xlink:href="linked.png" opacity=".4" /> 
    </symbol>
</svg>

Working:

<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewBox="0 0 400 400">
    <image width="400" height="400" xlink:href="linked.pdf" opacity=".4" />
    <image width="400" height="400" xlink:href="linked.png" opacity=".4" /> 
</svg>