I completed a postdoctoral period in China, where I had a number of issues with the administration and supervisor. In short, I felt exploited in many ways, from payment & funding restrictions to being constantly asked to provide unfair "favours" and contributions.
During my stay I was constantly asked for language revisions and suggestions on manuscripts. I am a critical person, and local culture is refractory to open criticism. Still, whilst my suggestions were generally ignored and apparently made others uneasy, lab mates would frequently ask for my opinion on manuscripts under preparation. They usually acknowledged my contributions in the acknowledgments section. Why they would ask for my suggestions when they didn't seem to like them always puzzled me.
However, since my departure they have published two papers I have never seen, acknowledging me for suggestions. Awkwardly enough, these are papers directly within my area of expertise that were surely under preparation in my presence, first-authored by close colleagues. (Meaning they probably refrained from mentioning the projects around me, at least in English).
Why would someone acknowledge a nonexistent contribution from a close colleague? Especially on a project that was apparently kept secret? It feels very awkward.
UPDATE: To clarify a key point lifted by all respondents, I am adding the exact text (specific info edited out) from the latest of them:
We thank Dr. XXX, YYY, United States; and Dr. [Scientist], ZZZ, China, for critical reviews of this manuscript.