This.

Our 2-year old Pyrenees 'Phoebe' loves being outside.  Sometimes I feel like I have to drag her in.  We don't leave her out at night though I think she would like to stay out.  She doesn't bark constantly but she has a very deep, loud, and imposing bark and I don't like disturbing our suburban neighbors.  When inside she very rarely ever barks so I guess she's just not on guard inside like she is outside.

My experience differs.

Attached File

My male, Niko.  We lost him last year, too young at 7 years old.   He was the most solid, fearless dog I have ever had, and that includes a Rott, Husky-Wolf Hybrid and Samoyed-Wolf Hybrid.   Great watch dog and very alert when it came to what was going on.  
Attached File

I had to get another Mini Schnauzer after he passed and ended up with two sisters, Nina and Mina.  While less alpha in some characteristics being female, they are awesome watchdogs.  Nina will bark at packages left outside our French doors.  The UPS guy hustles delivering shit and usually just walks up, drops, and leaves.  Both dogs usually hear the vehicle pull up, but by the time they get to the door he has gone.  They bark a little but then Nina gets really pissed.  It appears to be a combination issue of "can't you see some new shit is right outside the door" and "I can't see shit because of that shit sitting right outside the door".

Papillon
Cisco hears everything long before I do and he lets you know it.
Not near as yappy as the other small breeds.
He's very smart and agile.  Very loyal to his pet humans.
He's also is a world class mouser and rat dog.  
Great chick magnet too.
He some exercise on walks to get the mail and poopwalks but nothing too strenuous.

Neopolitan mastiff.

^^^. This. If you want a four legged alarm system there are none better. They're also good at keeping vermin away.