Couple of replays. Can't stress enough how important it is to scout around your base and different expansions, both natural and the outer ones.

http://www.sc2replayed.com/replays/79101-1...erg-lost-temple
http://www.sc2replayed.com/replays/79103-1...xelnaga-caverns

In the first one, opponent was a plat in solo and a diamond in another forgot. Well, sneaked in a pylon and when I noticed that it went unscouted for so long, it was time for a warp gate attack.

Second one, opponent tried expanding on a non natural expansion but luckily I managed to scout it. Otherwise they'd had way more mutas (my bad as well for not scouting again as well).

SCOUT SCOUT SCOUT!