haha, that sounds more like something an ESXP would do, admittedly, but without realizing it. My ISTP coworker would be the more likely one to whisper something like that. I think by talking, though.
Case in point: I made a joke about being a mooch since I had sat next to some people specifically to get a bit of food from them. ISTP dude acted like I was talking about another dude, though, (probably since nine times out of ten I'm the one sharing my own food) who mooches alot (and seems to be an ESTP as well). He eventually compared him to a vulture.
So later on moocher dude walks in and starts circling our table which has several items of food on it a couple times. I said, "There's the vulture!" and he just stopped and looked at me with a squinty questioning eye. I said, "Well you're circling the table..." And he nodded in understanding and went on his way.
ISTP guy later said when I made that statement he was like, "wow, I wouldn't have said that in front of him," and demonstrated how tense that made him. I just cracked up hardcore and said I'm just confident I can always redeem myself later.
Later on moocher dude started handing out all kinds of snacks to everyone, and ISTP dude was like, "I think it's probably because of what you said... but you know what's sad? It's too late to reverse that impresson." I chuckled and he said with his usual wide-eyed look of sincerity, "I know. It's sad but it's true."