I have been keeping a close eye on SC2 updates and as far as i know the new L.O.S (line of sight) rules in SC2 dictates that any unit on lower ground can't hit any unit on higher ground unless there is a spotter (allied ground unit) on high ground near the target or a flying unit .... (which is different from SC and SC:BW in which units reveal themselves when firing from high ground and low ground units get a reduction on their attack while hitting high ground units).

Now with that set in stone and without any announced changes to these SC2 L.O.S rules can someone (preferably from Blizz) explain how did the Hydralisks here take out that siege tank on higher ground here in this new gameplay video [ http://www.youtube.com/watch?v=h8q65mT3vpQ ] at (9:28) without any spotter (there were no zerg units on high ground near tank) or air units nearby (there was no overlord there either).

If Tanks reveal themselves when firing then the whole SC2 new L.O.S rules and high-ground-advantage thing Blizz kept promoting as a strategical element is pretty much useless and nonexistent... or are siege tanks an exception of that L.O.S rule !!!! (the only unit that reveals itself when firing from high ground) .. if so then why wasn't that mentioned anywhere and why would there be an exception to begin with !!!?

I'm hoping for a reply from a Blizz representative to clarify things but if anyone else knows for sure why did that happen so please point it out.

By watching the video you'll notice that there's a Xel'Naga watch tower just north of the spot where the tank was destroyed, near a Zerg expansion.

The Zerg player was in control of the Xel'Naga tower and its sight radius allowed him to see and attack the siege tank on the high ground with his Hydras.

No changes to the line of sight rules then, just another reason to love the watch towers :)