Ký hiệu ở trên đồ vật ẩn.

Vietnamese often does not use a separate copula in simple location sentences.

A very common pattern is:

• X ở Y = X is at/in/on Y

So in Ký hiệu ở trên đồ vật ẩn, the idea of is is already built into the structure. You do not need an extra word equivalent to English is.

Ở trên is a location expression.

• ở = at, in, located at
• trên = on, above

Together, ở trên usually means on or above, depending on context.

In many cases, English on is the best translation if something is physically on the surface of something else. If the thing is simply higher than something else, it may be closer to above.

Often, yes, but the feel changes a little.

• ở trên sounds more explicitly locational: is on / is above
• trên by itself can also work, but it is more compact

So ở trên is helpful when you want a clearly sentence-like structure.
Using only trên can sound shorter, more like a label, caption, or compressed written style.

Because Vietnamese adjectives and descriptive words usually come after the noun.

So:

• đồ vật ẩn = hidden object
• literally: object hidden

This is one of the biggest word-order differences from English.
English says hidden object. Vietnamese normally says object hidden.

Also, ẩn can behave like an adjective or a stative verb, so seeing it after the noun is completely normal.

It is understandable, but it may sound a little stiff or context-dependent.

A native speaker might more naturally say things like:

• vật ẩn
• đồ vật bị ẩn
• đồ vật được giấu
• vật bị giấu

However, in game or puzzle contexts, especially hidden object-type language, shorter phrases like đồ vật ẩn can appear and still be understood.

So the phrase is not wrong, but it is not always the most natural everyday wording.

Ký hiệu is a noun meaning something like:

• symbol
• sign
• mark
• notation

It is a somewhat formal or neutral word. In this sentence, it is the main thing being talked about.

So structurally, the sentence starts with the noun ký hiệu, and then the rest tells you where that symbol is.

Vietnamese does not have articles that work like English a/an and the.

Whether something is definite or indefinite usually comes from:

• context
• word order
• demonstratives like này or đó
• classifiers in some situations

So ký hiệu can mean a symbol or the symbol, depending on context.
Likewise, đồ vật ẩn can mean a hidden object or the hidden object.

This is very normal in Vietnamese.

Because classifiers are not required in every noun phrase.

They are especially common when:

• counting things
• introducing a single concrete item
• being very specific in speech

But in short statements, labels, instructions, or general references, Vietnamese often leaves them out.

So bare nouns like ký hiệu and đồ vật are perfectly possible here.

Yes, at least a little.

The most natural reading is that ẩn describes đồ vật, so the sentence is understood as talking about a symbol located on a hidden object.

But because Vietnamese is compact and often omits extra markers, short sentences like this can feel slightly compressed.

If you wanted to make it clearer, you could use a fuller version such as:

• Ký hiệu ở trên đồ vật bị ẩn.

That makes the idea of hidden object more explicit.

If, instead, you meant that the symbol itself is hidden, you would need a different structure.

That depends on the exact context, but possible more natural versions include:

• Ký hiệu ở trên vật bị giấu.
• Ký hiệu nằm trên vật ẩn.
• Ký hiệu ở trên đồ vật được giấu.

A few notes:

• nằm trên can sound a bit more natural than ở trên when talking about where something physically sits
• vật is often a little simpler than đồ vật
• bị ẩn / được giấu can be clearer than plain ẩn if you want to emphasize that the object has been hidden

So the original sentence is understandable, but a native speaker might choose a slightly different wording depending on style and context.