Very well Camouflaged Silo is hard to find from Spy Satellites or Aircrafts
Different camouflage types exist. For example Iran can use Camouflage Nets on top of silo and Desert Sand Glued on silo's retractable cover.
Detection methods that I remember (from the top of my mind):
1) Thermal difference (via satellite imagery), a bunker has not the same temperature than the soil elsewhere, nor the sand above it, at least for the parts that are not deeply buried.
2) During the construction phase ( with all the machinery involved).
3) Earth density (satellite imagery with other radars); if the earth has been moved, it doesn't present the same image
4) Air conduits are detected, so you have a bunker nearby
5) Some low frequency radars that can penetrate earth
And Soboka brought another one
then the oxidizer and fuel will have to be removed every 30 days or so, and the missile itself cleaned and refurbished
This involve a lot of activity, trucks,etc...
Saddam Hussein used to burry his tanks in Kuwait to prevent the US aircrafts to find them; they were virtually all destroyed, one by one, via thermal imagery because steel absorbed the sunlight and restitute the heat during the night.
The best defense against the American satellites is mobility.
But I think there might be another hypothesis behind the fact that Iran wished to build silos for the shahab. It takes a while before to master the technology behind the silos asccording to this officer. The shahab can actually be launched from a TEL vehicle but also from a silo. However designing a mobile launcher for a very long range missile, ICBM or not, is enormously complicated. It took a while for Russia to get the Topol-M. If the iranian engineers wished to have the option to build ultimatelly a very long range missile, like an ICBM, the safest option was to master the technology of silos in parallel rather than to gamble on an indigenous TOPOL. So the silos do not represent the backbone of the ballistic capabilities, which will be based on solid fuel missiles (sejil) and mobile launchers, but it's a first foot in the necessary technology for an ICBM.