Thanks!
I do this the other way around. The clue is embedded in the "boxed text" if you will (Step 1: DM describes the environment). This is my telegraphing - everyone gets it for "free."
You're welcome
Shame on me for choosing a quick an dirty example. Yeah, the steps I use are pretty much the same. I telegraph with "boxed text" in the same way, then go from there.
There's a tension, right?
On one side is "giving them enough info." On the other side is "keeping them wondering/guessing/engaged."
The "Yes, there's a secret door in the alcove..." or whatever comes after I know there is a character engaged in the task of searching for secret doors at the exclusion of other tasks and I have resolved any uncertainty as to the outcome with a game mechanic.
Requiring the players to engage is good! What concerns me are common posts like the OP about passive Perception & the Observant feat suggesting that it's common practice for DMs to cross-reference, for example, a secret door's or a pit trap's Perception DC with a PC's high passive Perception score and say something like,
"OK, Meliford, as you pass by the alcove your keen elven senses notice a secret door." 
I winced even writing that. Haha.
And it's not just the OP (don't mean to single you out!). I see this sort of thinking in the DMG, I see it in my gaming group sometimes, on Twitch streams, on the rare occasions I've joined a gaming event at my FLGS.
When I looked deeply at traps, Perception, and Investigation, I realized there is a whole group of "trap" monsters that get the False Appearance trait: Animated Objects, some Blights, Cloakers, Darkmantles, some Fungi, Galeb Duhr, Gargoyles, some Mephits, Mimics, Gray Oozes, Piercers, Ropers, Scarecrows, Treants, and Awakened Shrubs/Trees.
The wording is stark:
While the _______ remains motionless, it is indistinguishable from a _______. Compared to a hag's Illusory Appearance (which a creature can use an action to try to see through with Investigation) there is NO check that's necessarily involved with False Appearance. The implication, similar to deducing the presence/identity of a Doppelganger, is that you have to interact with the Galeb Duhr to figure out that it isn't actually a boulder.
So if a threat like a Galeb Duhr can be
indistinguishable from a normal boulder – NO check involved until interacted with – then why shouldn't a Rolling Boulder Trap or whatever have exactly the same False Appearance trait?
Taking a page from the recent UA traps article, if I were writing up countermeasures to a Galeb Duhr (conceiving of it similarly to a trap), I might write:
[SECTION]
Countermeasures:
- Dwarves know the true measure of stone, and by touching the "stone" recognize the Galeb Duhr for what it is.
- A spellcaster using a spell affecting stone, such as stone shape, on the "stone" causes the disgruntled Galeb Duhr to reveal itself.
- A Druid can expend a use of Wildshape to demand a Galeb Duhr within earshot assume its true form. Similarly, a Cleric of Nature or a Paladin with an Oath of the Ancients can expend a use of Channel Divinity to demand the same.
- All Galeb Duhr exert a subtle magnetic influence on surrounding rock. By using a compass or lodestone, a character can determine where the source of subtle magnetism is strongest – that will be the Galeb Duhr.
- The animated boulders of a Galeb Duhr are dim-witted and might be fooled by a character making a quick-witted argument and/or making a DC 15 Charisma check into revealing the Galeb Duhr's location.
- A character with tremorsense can use an implement like hammer or pick to strike stone within 30 feet of the Galeb Duhr, and pick up subtle vibrations that give away the Galeb Duhr's location.
[/section]
I'm not saying these need (or should) be extrapolated to this level of detail. I'm just demonstrating that dice rolling barely comes into play with these countermeasures. There's no Perception check involved. There's no passive Perception involved. A similar approach could easily apply to traps/secrets.
At least, that's my ideal. Sometimes, as a DM, I get tired or forget or a little frazzled. IMHO what would be *great* in times like those (or for new DMs) are some descriptive entries (whether for "trap" monsters or traps/secrets) with clues organized in a gradient format: boxed text ever so subtly hinting > clue moderately hinting.
