A general rule for writing (at least modern fiction that sells) is to try to never do anything that will wake your reader up from the trance of living your story.
Anything that is complicated and/or requires interpretation will break this rule. It will wake the reader up and force them to think that they are reading a novel and that they are now required to interpret what the writer is writing.
The same goes for anything that is hard to describe. Your unthankful job as a writer is to take the hard to describe and describe it, without breaking the above rule... or, if you're unable, write about something else. (I.e. write what you know... or do tons of research...)
He did the "I am watching you sign" before leaving-construction requires your reader to stop the immersion and think about what sign the person could be doing.
It's not showing the reader the sign.
I'd rather do something along: He pointed his fingers at his eyes and then at me. His gaze was dark and intense. This was definitely future problems coming my way.
I'm also not sure you should worry so much about if this gesture means something else in another culture than the one you're writing for.
For one, that's what translators are for. After all, the language is translated, obviously, so should the gestures be.
Also, maybe some people won't understand the gesture verbatim, but if you try to do a generic gesture or gesture description no one will get excited by it. (I.e. don't do it, it's bad writing...)