Well, I think that the level of heroism and fantasy needs to be taken into account.
"Most People" are commoners, or experts at best. Maybe warriors. The animals whomp their booties every single frickin' time. +5 to spot? No normal person (and by that I mean you) can ever get that much. Even an exceptional person (the proverbial 'eagle-eyed scout') won't get that or much close to it with the few paltry levels they pick up defending the outpost.
If the dog is traveling with the party, the dog (like any cohort) should get XP for the adventures the party undertakes and, like any party member, should increase in level. This increase brings skill points, feats, etc. It should be a
HEROIC dog. Thus, he'd increase alongside the adventurers, and while many wuold be able to be as keen at detecting something as him, many more wouldn't be.
I mean, I've snuck up on sleeping dogs before, and, according to D&D, I'm probably at best a second level expert (I'm being VERY generous here). If I can do it, something obviously more powerful than me shuold be able to, as well.
Yeah, dogs will rock any common burgaler or orcish scout, no problem. And if it wants to detect the supersneaky goblin rog7, then it needs a few levels, too, MacKenzie!
Basically, there should be no 'trump card.' If you want dogs to be super sensitive to super sneaks, give 'em a few levels. Otherwise, a +5 is more resepctable than any common human being could do, so they *are* better than you, a common human being.
That's the inherent heroism of the system. Just like a knife to the back won't always kill a HERO, a HERO can sneak past dogs and other canines like they're nothin', especially while they're sleeping. Unless, of course, those dogs have that same HERO status....in which case, an interesting scene occurs, because a heroic dog will usually sense a heroic person of the same level. Why should a barbarian/rogue be ALWAYS caught by a sleeping dog when he's a HERO? He's capable of cleaving plate mail and wields an axe that bursts into flame! He's not going to be caught sneaking around by a mutt!
Also, IIRC, being asleep just gives you a -8 to Listen and Spot. So even a sleeping dog can detect an advancing troop, just like a sleeping human could hear a twig snap....if they're heroic enough, if they've got enough levels...
Basically, D&D dogs are like real dogs...it's just that D&D monsters and characters are pretty much removed from reality by quite a leap and bound by virtue of levels and XP. If you want the dog to be able to conquer something that much removed from reality, remove it from reality yourself, slap a level or two of ranger on the thing (or just HD advancement), and call it an *encounter!*