How would you translate “social cue”?
Would you say something like “signaux sociaux”?
Notre couple d'amis restait assis, sans comprendre les social cues qui leur indiquaient que l'heure de leur départ était arrivé.
As I understand the term in English, “social cues” include not only nonverbal cues/hints, but also verbal ones, including tone of voice (link to reference.com & its definition of “social cues”).
This “nonverbal and verbal” definition also seems to be the case in French, at least according to the site TA@l’école, which uses “les indices sociaux” in conjunction with L'Association canadienne des troubles d'apprentissage’s definition of the term “perception sociale”:
“[la] capacité d’interpréter les situations sociales, par exemple en «lisant» l’expression du visage, le ton de voix, le langage corporel et d’autres indices verbaux et non verbaux. Un déficit au niveau des perceptions sociales qui guident le comportement peut entraîner des problèmes sur le plan des aptitudes sociales.”
Granted, an "indice social" could be confused with a “social index” or “social indicator” as that term is used, at least in France, to measure and compare the social backgrounds of students and their individual schools and regions (see this snuipp.fr discussion of this type of “Indice Social”).
However, I think using "indices sociaux" would still be a good way to cover the “and verbal cues/hints” part of the definition and I think the context surrounding the term’s use, at least in Canada, would make it clear enough when the overall concept of “social cues” is being discussed, both in general and academic circles. .
Although I think your suggestion, “signaux sociaux,” also captures well the definition's neutral nature (being either verbal or nonverbal), I feel that it might be more likely to be confusing, on both sides of the Atlantic, than my suggestion when discussing the general concept of the term. I feel this way because “signaux sociaux” seems to be used primarily in the world of social media and networks (see this conflicting use of “signaux sociaux” in this arobasenet.com article).
All that being said, however (and with the importance of context still fresh in mind), I find that your specific example, by so clearly stating the desired purpose of the missed cues/hints as it does, would not require, either in the French or in its English version, the inclusion of the adjective "sociaux/social."
Therefore, using "cues/hints" alone would, in my opinion, be sufficient in a plausible English translation of your example ^^(see my rough one, below) and, similarly, simply using my suggestion ("indices") or yours (signaux") would be sufficient in your French example.
^^("Our two friends remained seated, having failed to get/take the cues/hints designed to let them know that it was past time for them to leave.)
In fact, in your specific example, exactly as it is written using the verb "indiquer," I would actually prefer "signaux" alone over "indices" alone, if only to avoid using "les indices qui leur indiquaient," which I find somewhat awkward if not, for lack of a better reason, somehow redundant.
"Notre couple d'amis restait assis, sans comprendre les signaux qui leur indiquaient que l'heure de leur départ était arrivé."
About your specific sentence, here is what I would use:
Notre couple d'amis restait assis, sans comprendre la gestuelle de leurs hôtes qui sous-entendait que l'heure du départ était arrivée.
Notre couple d'amis restait assis, sans comprendre le comportement de leurs hôtes qui sous-entendait que l'heure du départ était arrivée.