I describe things that are notable with the assumption of reasonable competence on the part of the adventurers. Perhaps instead that pile of carcasses is significantly larger than what they've seen before or more thoroughly picked clean or suspiciously free of any kind of putrid smell or there's a large four-toed footprint in the mud near it, partially filling with water. Now, "What do you do?" There may be no encounter here whatsoever. It might simply be an opportunity to explore.
In my experience, this is not how it plays out. Really, I want the players to be "on alert" in the sense that they're paying attention to their surroundings and interacting with them in meaningful ways. As well, not every description of an environment results in an encounter (see above).
Explore what though? Are you creating a new adventure on the fly? I think that nearly every DM description should result in something worthwhile though, not necessarily an encounter.
I tend to dislike what I call pedestrian roleplaying and pedestrian exploration (although having totally empty or non-interesting rooms in a dungeon are fine, I just tend to only have some of those).
If I describe some carcasses, I want there to be an in game reason that this information is important later on, even if there is no encounter at the moment.
I also do not want stupid little "the Bard hits on the barmaid" roleplaying sessions. Or, angst roleplaying sessions. These might be fun for some players, but when 5 players are sitting there listening to this type of thing while one players is just bull____ing, it's a waste of gaming time IMO. If the Bard is hitting on the barmaid in an attempt to get specific information that he thinks she might have, that's a beast of a different color.
Some DMs add in pedestrian stuff, just so that the players and they can now suddenly create a brand new DM unprepared story out of whole cloth on the fly. I'm not that type of DM. There is enough story getting created without having to jumpstart it with pedestrian stuff. Granted, all DMs through stuff in on the spur of the moment that they just thought of (the Duke's son shows up riding his horse as the PCs are heading to the market), but I want it to be meaningful stuff, not just a chat with the Duke's son that has no real effect on the game at all. Some DMs might feel that these types of pedestrian events make the game feel more organic and real, but there are only so many gaming hours per session (and at the moment, we are lucky to be playing every other weekend), so I prefer to cut to the chase.
Further, even if the players are engaged in metagame thinking, I'm okay with that so long as they take actions in the game to verify their assumptions. Metagame thinking is only a problem when a player's expectation turns out to be wrong in a dissatisfying way, such as when they assume an encounter is "level appropriate" then proceed to get their ass kicked (see DMG, page 235). But that's on the player. I can't control how other people think or how they make decisions for their characters. I can only provide them with opportunities to engage with the three pillars of the game as they see fit.
Taking actions is usually not an issue. Getting players to take DM desired actions sometimes is.
DM: "You see that the river goes further inland into some hills." (the DM knows that there are caves and adventure in the hills, but the caves cannot be seen from the current location)
Player: "Ok, we cross the river and continue our journey."
As a DM, I am totally ok with this decision. Typically, I am not going to be bothered by the fact that the players did not follow my subtle hint, nor am I going to add a bunch of overt hints, just to make sure that they pick up on my clues.
I probably would not start this same conversation out with:
DM: "You see that the river goes further inland into some hills. The river is wide and deep here and will take longer to cross. Maybe upstream, the river might be less deep. And you see a rock feature on one of the hills, but you cannot make it out from here."
Some DMs will make environmental descriptions like this latter one, practically hitting their players over the head with the direction that they want the PCs to head towards. I'm not one of those DMs. I do give hints and indications (and even an occasional railroad), but mostly if the players do not pick up on it or blow me off, I don't care too much and I usually am only overt with things that I feel should be overt. I just also typically have a backup plan (what happens if they just cross the river) and go with that.
As an example, one of my most recent "railroads" was to have two of the PCs have a dream from deities to get them to go in a certain direction and look for certain magic items. One dream was from the goddess Mystra for one PC, one was from the goddess Tymora for another PC. This to the players (who btw, I took off to the side separately, so the other players do not know why these two PCs are voting for a given group direction) seems like obviously railroading because what PC is going to not follow the wishes of a deity? In reality, both of these were set up by one of the BBEG's lieutenants drugging the two PCs before they went to bed in the inn, and then entering their rooms and casting an illusion of what he wanted them to see and do. Later on in the adventure, those two players will find out that this was all part of the evil plan and had nothing to do with deities. The NPCs were getting the PCs to go find specific magic items that the NPCs knew about and wanted, but it was too dangerous for the NPCs to get those items themselves. Yup, a railroad of sorts, but one that was based solely on things that I had already determined existed in the campaign and things that happened earlier with the PC / NPC interactions. The players of those two PCs could have decided to ignore the dreams (or could have been outvoted by the other four players), but the reason the NPC picked deities in the first place was because most creatures in a fantasy world like this would never think to ignore a dream sent by a deity.
