they watch the package, not the table they were at ten minutes ago, not the coat room, they do make sweeps of the building and each room they enter beforehand, and random sweeps of the building while they're there but the most efficient thing is to stay with the package.
@ kmarion, if you look in most major city phonebooks, you'll find a listing for a local sec service branch, each one with a few cars, some of them with limos on ready. though alot of times the vehicles are convoyed from nearby or stuffed in a c-17 and flown ahead. that vid made my head hurt, but usually the sec service likes to keep a pretty low profile. there could be one next to you tomorrow and you'd never know, as the tint is so dark it makes looking for lights nearly impossible. they don't just do guard service, but they handle credit card fraud, pyramid schemes, and counterfeiting as well, justifying stocking "fleets" in each major city. you won't see an address in the listing, but it's there.