Why would a request seemingly by-pass the F5

Thanks for your replies. Yes the Virtual does have a one-connect profile on.

@abdul rahman - what may be in my dns sometimes? Can you elaborate a bit on this please?

Here is the iRule in quesiton (some urls changed due to sensitivity)

priority 101

when HTTP_REQUEST
{
    log local0. "Working on: [HTTP::uri]"
    if { [TCP::local_port] equals "443"  } {
        set scheme "https"
    } else {
        set scheme "http"
    }
    if { [HTTP::path] eq "/" } {
        if { [HTTP::query] contains "Silo=Type" } {
             Silo Override
        } else {
             Set variable defaults
            set client_ip "0.0.0.0"
            log local0. "client_ip set to 0.0.0.0"
             Check for existing LocaleCookie
            if { [HTTP::cookie exists "LocaleCookie"] }
            {
                log local0. "LocaleCookie exists... extracting data"
                 Extract country from locale
                 example: market=GB&locale=en_GB&language=en&country=GB
                set country [findstr [HTTP::cookie value "LocaleCookie"] "country%3D" 10 2]
                 Extract the language (for WCS sites only)
                set language [findstr [HTTP::cookie value "LocaleCookie"] "language%3D" 11 2]
                log local0. "set country = $country & language = $language from LocaleCookie"
            } else {
                set client_ip [IP::client_addr]
                log local0. "client_ip set to $client_ip"
                 Get the client IP from the XFF header if available (testing override)
                 Un-comment to enable
                if { [HTTP::header exists "XFF"] } {
                    log local0. "XFF present - values: [HTTP::header values "XFF"]"
                    set client_ip "[getfield [HTTP::header values XFF] " " 1]"
                    set client_ip [getfield [lindex  [HTTP::header values XFF]  0] "," 1]
                    log local0. "client ip extracted from XFF is $client_ip"
                }

                set country [whereis $client_ip country]
                 Use country as language (for WCS sites only)
                set language [string tolower $country]
                log local0. "set country = $country & language = $language from LookUp"
                if { [string length $country] == 0 } {
                    set country "UNKNOWN"
                    log local0. "country set to UNKNOWN"
                }
                if { [class match -- $country equals EU_Countries] } {
                    log local0. "Found EU country: $country"
                    set country "EU"
                }
            }
            switch $country
            {
                "GB" { set location "/locale/country?country=GB" }
                "ZA" { set location "/locale/country?country=ZA" }
                "IE" { set location "/locale/country?country=IE" }
                "US" { set location "/locale/country?country=US" }
                "CA" { set location "/locale/country?country=CA" }
                "EU" { set location "/eu/en" }
                "FR" { set location "/fr/$language" }
                "IT" { set location "/it/$language" }
                "ES" { set location "/es/$language" }
                "UNKNOWN" { set location "/country-selection" }
                default { set location "/locale/country?country=AV" }
            }
            log local0.debug "XFF=[HTTP::header values X-Forwarded-For] IP=$client_ip C=$country LOC=$location"
            log local0. "redirecting to: $scheme://[HTTP::host]$location"
            HTTP::redirect "$scheme://[HTTP::host]$location"
            return
            event disable all
        }
    } else {
        set locale [string range [HTTP::path] [expr {[string first "/" [HTTP::uri]] +1}] 8]
        set url [string range [HTTP::uri] [expr {[string first "/" [HTTP::uri]] +10}] end]
        set checkurl [string range $url 0 [expr {[string first "/" $url] }] ]
        if { ([class match -- $locale equals cs_markets]) } {
            set newlocale [class search -value cs_markets equals $locale]
            log local0. "Got locale: $locale - translated to newlocale: $newlocale"
            log local0. "Got remaining URL: $url | checkurl: $checkurl"
            if { not ([class match -- $checkurl equals cs_exception_urls]) and 
            $url != "dummy-url1" and
            $url != "dummy-url2" and
            $url != "dummy-url3" } {
                log local0. "Redirecting to $scheme://[HTTP::host]/$newlocale/$url"
                HTTP::redirect "$scheme://[HTTP::host]/$newlocale/$url"
                log local0. "Redirecting to $scheme://[HTTP::host]/$newlocale"
                HTTP::redirect "$scheme://[HTTP::host]/$newlocale"
                event disable all
            } else {
                log local0.  "Either class (not) matched, or xxxx URL : $url"
            }
        }
    }
}