There are tons of incidents with intruders on military bases where I am from. They are atrractive targets for criminals like thieves, too. Guards aren't a very effective deterrent.
Also, everyone I talked to who found suspicious activity while on guard duty confessed to me that they were terrified in the moment even though they were trying to project authority on the outside.
Depends on the base. Most bases have controlled entrances and patrolled perimeters. I do know of some that are more or less open or only require a photo ID for entry, but those don't seem as common. Or maybe times are changing.
Are you in the US? All of the US military bases I have ever been on either have far tighter perimeter security than anywhere bar maybe a high security prison, or have generally pretty good perimeter security and are so far out in the boonies that geography itself limits access.
Also, everyone I talked to who found suspicious activity while on guard duty confessed to me that they were terrified in the moment even though they were trying to project authority on the outside.