Perception and Investigation

Something I've been doing wrong with 5e since I started:

The purpose of the Perception skill (technically a Wisdom (Perception) check) is to determine what the character's senses can detect - does the character see the tripwire, smell the gas, hear the voices in the other room, taste the poison, feel the gust of cold air... or intuit the presence of magic, hearken the the presence of a god, or similar.

The purpose of the Investigate skill (technically an Intelligence (Investigate) check) is to assemble known facts into conclusions: this funny smell combined with the lantern flame turning that colour means we're about to encounter flammable gas; this taste could just be almonds... or cyanide; the tripwire, coupled with these holes in the wall, indicate a poison dart trap... and so on and so forth.

This means a few of things:

• When the PCs enter an area, the DM should describe all the things that they see, hear, smell, etc, depending on their passive Perception scores... but should not indicate what these mean.
  
• When a PC searches an area, the DM should first ask how they go about doing that (in broad terms), and then call for a Perception check, and then describe what they now see (but, again, should not indicate what these mean).
  
• PCs should probably also have a "passive Investigate" score, calculated in the same way as for passive Perception. And based on that, the DM should probably also explain how some of the things detected fit together.
  
• When PCs search an area, the DM should probably also call for an Investigate check (in addition to the Perception check) and use that to describe how some of the things detected fit together.

To give an example of something that's likely to come up in my next session...

Player: I'll examine the clock. Is there anything of interest there?
Me: It's a very large, very ornate clock. It's a very modern construction, but made to look considerably older. The mechanism appears to be extremely complex. It also looks like the back of the clock is recessed into the wall itself.
Player: I'll open it up to take a closer look at the mechanism.
Me: Make a Perception check, and also an Investigate check.
Player: I rolled a 17 for Perception, and 13 for Investigate.
Me: You note that several of the components are free moving - they don't appear to be part of the mechanism for the clock itself. You also note that the back of the clock is hinged. You don't know what this means.
Player: Ah, there must be a secret door. Can I check for traps?
Me: You've already checked. You didn't detect the trap.
Player: The trap?
Me: {grins}
Player: Okay, can I move the mechanism to open the door?
Me: Sure, roll a Dex check. Your proficiency with Thieves' Tools applies...
Player: 24.
Me: Manipulating the device is easy. You move the various levers around, and... well, you know I said you didn't detect the trap?
Player: Yes...
Me: Click...

I guess I should maybe provide some context to this: in the example given, the clock contains both a secret door (DC 15) and a trap (DC 20). The player rolled well enough on Perception to notice the moving parts associated with the first but not the second, but didn't roll well enough on Investigate to understand either. If the latter roll had been a 15 or higher, I would have told him that this was all part of a secret door.

You'll notice that in the example I didn't allow the player a second roll to search specifically for traps. To get a second roll, the player would have needed to significantly changed the scenario - by smashing apart the clock, by taking an age to break it all down, or similar.

Lastly, I've adopted the "click" rule described by the Angry GM: when a trap is triggered the PCs have an immediate chance to declare an action. They don't get told the context, and so effectively have to guess - dive to the side, jump straight up, stop breathing, etc. If they guess right, they gain Advantage on the resulting save (or the trap has Disadvantage on the attack); if they guess disastrously wrong they suffer Disadvantage; in most cases, or if they dither, they just get hit normally. (In extreme cases, Dis/Advantage may turn into auto-success/failure. But that's less common.)