The police are allowed to look at anything else that a regular citizen can look at. Officers legally in a place do not need a warrant to observe everything around them. While this is generally called the “plain view doctrine,” nothing prevents officers from using all of their senses to listen, feel, or otherwise observe, and if the incriminating nature of the object is obvious, seize it. (For the rest of this post, I will use the sense of “sight,” as a shorthand for any of the senses.)
Once officers see contraband, assuming they are legally present, they can seize it. The incriminating nature of the evidence has to be immediately apparent. Any further sleuthing by the officer is not allowed, such as moving a stereo to check its serial numbers. But something like illegal drugs, which are always illegal, can generally be seized.
An officer can even seize contraband in situations where they are on private property. For example, an officer does not need a warrant to seize a marijuana plant growing on someone’s property, even if there is a wooded fence keeping intruders out. Because anyone could see the plant, the police can also seize it or use the information for an arrest warrant.
The phrase “plain view,” can be a little misleading. Often, the incriminating item is not in “plain” view, but it is visible. For example, officers can press their noses to a car window and contort their heads to look for evidence. If an officer saw a loaded firearm, that would qualify for the plain view exception, even though most people would believe that the officer’s necessary contortions show that the firearm may have been in view, it was not in plain view. The bottom line is that an officer is not forbidden from seeing what anyone else can see, and anyone else could do the same thing, obnoxious manners though it would be.