Found this on another site, I do not take credit for this! Just thought you guys might want to see it, hope it helps
- in the waterfall level near the pillar (green tree, first warp zone),
- in the windmill level near the windmill (orange tree, air zone),
- in the forest near the transforming cabin (green tree, outside rainy zone),
- and outside the octopus-looking building containing the revolving model of the solar system and the counting cube (pink tree, temple/science zone).
(These locations are shown on the walls of the fez-wearing elder's house [across the bridge] in Gomez's home town.)
After speaking with each owl, they will fly back to the owl stage in the rainy zone. When all four arrive, a panel reveals a door behind the owl statue. (The current glitch lets you access the hidden room anyway: stand in front of the entrance, rotate the room twice, and press up, and you'll enter.) The room won't be completed until the main secret (four owls) is finished and the hidden room is visited