Mysterious visitor to hidden PHP page
Solution 1:
Websense? Websense is in the business of classifying URLs and looking for "naughty" things on the Internet. Their products usually show up in corporate environments.
I'd bet that you accessed your secret page of HTTP from a company that has Websense installed and they automatically added the page to their (presumably gargantuan) list of pages to troll checking for porn, warez, forums, etc.
As for the varying header, I'm guessing their robot has all manner of possible banners to choose from an intentionally changes them up to mask itself from analysis and pretend it's not a bot. In fact, a quick Google search of FunWebProducts websense all but confirms the theory.
Solution 2:
The IP address range belongs to Websense. You may have one of their product running.
$ whois 208.80.194.0
[Querying whois.arin.net]
[whois.arin.net]
NetRange: 208.80.192.0 - 208.80.199.255
CIDR: 208.80.192.0/21
OriginAS: AS13448
NetName: WEBSENSE-NET2
NetHandle: NET-208-80-192-0-1
Parent: NET-208-0-0-0-0
NetType: Direct Assignment
RegDate: 2007-07-25
Updated: 2012-03-02
Ref: http://whois.arin.net/rest/net/NET-208-80-192-0-1