Instead of doing a capture would it be easier to just use something like {% if page.modified %}<p class"notice">{{ page.modified | date: "%Y-%m-%d" }}{% end if %}

This way. If a date is specified in the front matter it displays on the post. If it's not then it doesn't and you don't have to mess with assigning a true value to hideupdated.

I would probably place the updated date in the side bar below the published date too so everything is clean and the content stands by itself. Let me see if I can work on something...