If you have your target pinned, otherwise restrained, or unconscious, you can use rope to tie him up. This works like a pin effect, but the DC to escape the bonds is equal to 20 + your Combat Maneuver Bonus (instead of your CMD).
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If you are tied up, you have the pinned condition.
A pinned creature is tightly bound and can take few actions. A pinned creature cannot move and is denied its Dexterity bonus. A pinned character also takes an additional –4 penalty to his Armor Class. A pinned creature is limited in the actions that it can take. A pinned creature can always attempt to free itself, usually through a combat maneuver check or Escape Artist check.
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Someone who is pinned can only attempt to free themselves (or take mental/verbal actions), which means that they can't take the Standard action required to draw a hidden weapon.
The DM could rule that you get a circumstance bonus to the Escape Artist check if you have a blade on your body, but they still require a skill check to cut through the bindings.=====
You can hide a small object (including a light weapon or an easily concealed ranged weapon, such as a dart, sling, or hand crossbow) on your body. Your Sleight of Hand check is opposed by the Perception check of anyone observing you or of anyone frisking you. In the latter case, the searcher gains a +4 bonus on the Perception check, since it’s generally easier to find such an object than to hide it.
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Also, you could just search them over and over with the +4 perception bonus from frisking. They'd get a Sleight of Hand check to prevent you from noticing, probably with a +4 bonus themselves if if their blade is a dagger and they're wearing a cloak.
But it is a classic trope, so I would reward them for successfully secreting a blade and just allow them to escape in 1d4 minutes of cutting at the rope.
Otherwise it appears the rules prevent this scenario from happening altogether.
This doesn't really address the rules loophole, but I will say that my party had a habit of whacking anyone who was tied up until they fell unconscious (non-lethal damage) and doing so repeatedly to make sure they stayed unconscious but alive. That and breaking their hands (nasty casters).
You can never be too sure with your quarry.
As Magda Luckbender states this is in the realm of GM’s option.
The description of the pined condition does not state you can only attempt to free themselves. What it says is that there actions are limited and that they can always attempt to free themselves. It does state that you cannot cast spells with somatic or material components, but does not specifically list any other physical actions that are prohibited. Because it says your actions are limited, but does not really state exact details it is again in the realm of GM’s option.
How I would handle it would be to have them make a sleight of hand roll to draw the object and then if the succeeded give them a +2 circumstance bonus on escape artist to free themselves. This is assuming the blade is fairly small, if it is larger than the bonus may be higher, but the chance of it being found would be higher.