24-Hour Surveillance

I saw this on someone's gate recently: "This property is guarded 24 hours a day!"

24 hours!!

When I saw it, I thought: what does "24 hours a day" actually add?

If it just said "This property is guarded", would the meaning be any different? No, not at all.

I've never seen a sign that reads: "Warning! This property is guarded 8 hours a day!" As a burglar, I'd think: oh... I'll just come back later then.

The "24 hours a day" is essentially an intensifier. This property is really guarded. And there's an exclamation mark too. So yes, it's being watched very, very seriously indeed.