Idiom/word for seeming misleadingly encouraging

You could use the expression in disguise:

Upon deeper contemplation, the news struck me as a warning in disguise.

or

Upon deeper contemplation, the news struck me as a desguised warning.

To disguise means

to give a new appearance to a person or thing, especially in order to hide its true form. If people, objects, or activities are in disguise, they appear to be something that they are not, especially intentionally:

• He claims that most Western aid to the Third World is just colonialism in disguise. (Cambridge)

You could use façade in that blank, but I would rephrase it as ‘Upon deeper contemplation, the news struck me as a façade hiding a warning’.

2. SINGULAR NOUN

   A facade is an outward appearance which is deliberately false and gives you a wrong impression about someone or something.

They hid the troubles plaguing their marriage behind a facade of family togetherness.

[Collins Dictionary]

That would be a veiled warning.

Here's the relevant definition veiled from the OED, with a couple of usage examples:

veiled, adj.
2. figurative. Of immaterial things.
b. Not openly declared, expressed, or stated; implied or inferred. Also: covert, disguised.
_{1934   Stevens Point (Wisconsin) Daily Jrnl. 8 May (City ed.) 1/7   His statement..carried a veiled warning that if anything happened to him because of his imprisonment the government would be held responsible.
2015   Guardian (Nexis) 22 June   It was hard to know whether the comments..were a joke or a veiled threat.
Source: Oxford English Dictionary (login required)}

A pseudo warning?

pseudo [soo-doh]
adjective;
not actually but having the appearance of; pretended; false or spurious; sham.