Idiom/word for seeming misleadingly encouraging

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)}