@kabum @butonic I've found the problem - the search results box looks fine with the top set to 20px if you are on the 'files' app page, because of this 
/apps/files/files.css, line 64

    tbody tr { background-color:#fff; height:2.5em; }

This gives the first row of the searchresults div a height of 2.5em, as opposed to 0, hence why on most other apps the first line of search results are obstructed by the header. 

To work around, we could give the row a height of 2.5em in the search/results/css file, or we could fix the styles css, make it specific to the 'files' app, and not affect the 'core' css.

As a side note - does anyone have any preferences/suggestions when building apps as how to style elements? My prefered way, to prevent messing up the core style, is to prefix all my styling rules with '#content', which could also be a bit messy (whilst there's no easy way to implement SASS).