Under the plain view doctrine, anything that is in plain view and is clearly evidence of illegal activity may be seized without a warrant during a traffic stop.
There are two aspects to this rule that must be considered. First, the evidence must be in plain view. The police cannot (without probable cause) search such places as the trunk of a car. The evidence must be out where it can easily be seen. Second, it must be immediately apparent that the thing being seized is illegal. For example, a bag full of marijuana is clearly illegal and can be seized. On the other hand, a manila envelope that is closed and sealed is not clearly illegal and therefore cannot be seized.