BB have actually taken steps to ensure that collectibles don't appear behind rocks - there were a few locations they used to appear where they couldn't be found, but they don't appear there any more.
It's much harder to implement a system that prevents a collectible appearing behind a building - I believe they appear anywhere it's possible for you to place a road (or already have), yet they don't tend to appear in closely packed areas, or behind buildings very often. When they do appear behind buildings, it's usually a larger building - if a normal building was in the same location, the collectible wouldn't be hidden.
So, to find collectibles:Following those tips above, it's rare I miss any...
- Run your mouse through all trees on your map
- Run your mouse over any large buildings
- Carefully check immediately in front of every building
- As Saraptheold said, remember the locations where they were 'hidden' and double-check those places each time