Dealing with overly cautious players!

[Empirate]

Cavall, to be honest I believe you totally missed the OP's point. He's not trying to "keep a way to screw over" his PCs. He's trying to improve the flow of the game, which is currently bogged down, not by "ingenuity and planning", but by irrational and uncalled-for paranoia and rolling of checks on every single square of terrain. The OP's statement "I've never been the kind of DM who lobs a TPK at them whenever they let their guard down" makes that very clear.

I really think Jack Simth nailed it in his response. You either go about it out of game (which seems to pose some problems), or in game - for which there are several solutions. Not using hidden stuff at all unless clear hints are provided seems a good and fair way of going about it - especially paired with time-sensitive missions.

You could even make it really obvious, such as in a fairy tale-like way. An important NPC could give them their mission and say:
"The whole of the Caverns of Short Time will look horribly dangerous to you, but that is a magical effect to make you waste time! Don't fall for it!"
Then put them under severe time pressure in the Caverns, and make sure there are no hidden things to kill them. Should bring the point across.

After such an adventure, make sure to always provide clear hints on the presence or absence of traps. It's OK to be careful if you have good reason - but it's also OK to rely on your gut feeling that there is no danger when nothing hints at it. Remember, it takes only one nasty attack out of the blue to make your players paranoid again, if it comes too soon.

 

[Dandu]

Clearly someone should start bringing alcohol to the gaming table.

 

[El Mahdi]

Along with assuring them that you aren't going to practice "Gotcha" DM'ing, something else that might help (so that they aren't searching every 5' square of terrain), is this:

When they enter a new room or area, have them describe what they are going to do to search the area, and then have them make 1 roll to cover the entire area (per person). Might cut down on the amount of rolls that they are needing to make and speeding up gameplay.

If that doesn't help, and only as a last resort, modify the spells to require material components, and require the Rogues skills to require some components (like talcum powder/dust, brushes, etc.). Then limit access to, or the ready supply of, the components. That way they have to use their detection skills and spells much more sparingly, as an expendable resource. But again, only as a last resort.

 

[Starbuck_II]

Don't use traps and tell them that.

No need to search for something you promise to not use.

The issue is if you want traps:
But if you use them why shouldn't they search for them?

 

[Dukthulhu]

Empirate nailed it. I don't want them to start getting hit by traps all the time, I just want them to stop searching everything they come across. I'm fine with 10%, 50%, or even 75% of their obstacles being thoroughly examined, but that's not what happens.


Me: "You come across an open doorway. The oak slab that previously filled the portal lies askew, barely held on by a single, twisted hinge. From the opening--"
Beguiler: "Trapfind."
Wizard: "Detect Traps."
Sorcerer: "Detect Magic."
--
Beguiler: "I don't trust this guy."
Me: "The merchant looks friendly enough. His goods appear solid and trustworthy, and he doesn't seem to have any ulterior motives that you can think of."
Beguiler: "Sense Motive."
Wizard: "Detect Evil."
Sorcerer: "Detect Magic."
--
Me: "The hallway ahead looks completely empt--"
Beguiler: "Trapfind."
Wizard: "Detect Traps."
Sorcerer: "Detect Magic."

Yeah, it's pretty much like that.

 

[jonesy]

Hang on, they don't even let you finish describing the scenario? If they start investigating the open doorway before the DM finishes and the description ends with "..there are orcs at the end of the hallway" I'd totally have them get jumped by the orcs while they're still ogling the door. The description is free time. It's what the characters see when they enter an area. If you don't want to hear that, then you don't care to examine it either. And the merchant? If he happens to be magically attuned and notices his customers casting spells on him he just might be inclined to deny them business. Or maybe have him point to a sign on the wall that says something like 'anti magic zone' or 'this establishment under permanent detect evil'.

 

[frankthedm]

Me: "You come across an open doorway. The oak slab that previously filled the portal lies askew, barely held on by a single, twisted hinge. From the opening--"
Beguiler: "Trapfind."
Wizard: "Detect Traps."
Sorcerer: "Detect Magic."

Trapfind take time [one full round per 5' area. 2 minutes to take 20] and spells take up resources...

In fact where is this detect trap spell coming from? It is not on the normal wizard spell lists! Only similar spell is a cleric spell http://www.d20srd.org/srd/spells/findTraps.htm and that one is not that strong.

Nystuls magic aura can help

 

[Dukthulhu]

...I think I need to have a talk with my Wizard.

 

[LordXxXErebus]

Anti-magic the area

 

[frankthedm]

Beguiler: "I don't trust this guy."
Me: "The merchant looks friendly enough. His goods appear solid and trustworthy, and he doesn't seem to have any ulterior motives that you can think of."
Beguiler: "Sense Motive."
Wizard: "Detect Evil."
Sorcerer: "Detect Magic."

Sense motive should be rolled by you in secret.

Casting a spell is a very hostile action. If someone tries casting a spell when non allies are not expecting them to, call for initiative, because merchants are calling for the guards.

Also if the players routinely interrupt you, you might want to consider asking them to leave or reducing their XP awards for bad play.