Touch it, walk on it, or shoot it. The majority of doors on Stroggos open as you approach them. If one doesn't open automatically, seek a button, floorplate, or key. Most secret doors open when shot. Others open by activating floorplates, buttons, or levers.
Specific symbols indicate the exit to a given area.
The Strogg marked off areas to indicate a one way passage. Once you leave a unit complex you cannot return. The Strogg have concealed military supplies in different locations within the city. Recon your surroundings closely. Sometimes the slightest variation in the environment could mark the entrance of a secret area.