I have an XML document containing a long list of Byte elements, e.g.,

I have a string of hex values, e.g.,

04000000FF

You can see that the string is contained within this series of Byte elements:

What is an efficient way to solve this problem?

let $req := '04000000FF'
let $len := string-length($req)
return
    head(
        for sliding window $w in /Bytes/Byte
        start $b1 at $s when $b1 = substring($req, 1, 2)
        end at $e when $e - $s + 1 = $len idiv 2
        where string-join($w) eq $req
        return $s - 1
    )