The plain view doctrine is often cited in criminal cases when establishing how evidence was found or why a search was carried out. It is very important to understand how this works and how it can be used.
Essentially, the doctrine just says that police have the right to carry out a search and seizure if they see something that could be evidence in plain sight. They don't have to go back and get a warrant, as they typically would.
For example, if the police suspect that you were involved in an armed robbery, they may come to your apartment to talk to you. Now, you don't have to let them in, but you may open the door when they knock to speak to them. If they spot a gun on a desk that looks like the type of gun used in the robbery, they could then enter the apartment and take the gun, even though they did not have a search warrant for the apartment.
This is simply one example, but you can see how this could be applied to drug cases and many other cases.
Now, in most situations in which the police enter a dwelling without a warrant and find evidence, you may be able to ask for the evidence to be thrown out on the grounds that they never should have been there to find it. Anything stemming from an illegal search is known as fruit of the poisoned tree. That's what makes the plain view doctrine so important, as it means even evidence collected during a warrant-less search can be used.
Be sure you know all of your legal defense options in Maryland if evidence collected in this fashion is being used against you.
Source: Cornell Law School, "plain view doctrine," accessed May 05, 2016