Dungeons and lairs are filled with objects and decorations. Not only should you use these, but make it not obvious.
A small fountain sits in the center of the courtyard.
Type: magical; Perception DC 29; Disable Device 29
Trigger Proximity; Reset None
When a PC gets within 10' of the fountain, the water churns and splashes outward. When a PC is splashed by the fountain they are imbued with 1d10+7 temporary hit points that they are not aware of. These hit points will be lost first. To the PC, it will be as if he was not damaged until they run out. However, once they run out and the PC actually takes damage, he will also take additional damage immediately for the amount of temporary hit points he was granted. Fountain Splash(DC 21 Reflex Save avoids); multiple targets(all targets within 10')
This one will work especially well if you don't tell the PC's anything happened when they get splashed. Just secretly roll up the temporary hit points, and next time they are in combat use them. You can describe the action, "You get hit with an axe. It penetrates your armor and bites into you flesh, but you seem to shake off the wound as if nothing happened." They probably won't know why they aren't taking damage, but they won't question it. However, once the hit points run out, make sure to describe what happens. "You take a glancing blow, but suddenly, wounds appear and begin to bleed profusely as you slip into unconsciousness".
They still may not put it together, so you may have to use this again, and again... At any rate, once they figure this out, every time something minor happens, the players will suspect the worst. You can use this to build tension or to bring another layer of action to life in your adventure.