Gentleman of the instructing variety,
Posting this as as outsider (P/UT PPL, hope you don't mind)
FFF,
I can't see how the Guy can have a pop at you, your eyes can not be everywhere, instructing or not.
I'm sure that anyone doing internal checks would have a reasonable situational awareness (through perhaps glancing up occasionally and maybe becoming aware of new noises around them) but not to the extent of noticing that the Guy in front is throwing oil.
If you notice, all the better, if not, unlucky for the person with the fault.
IMVHO, No one did anything wrong.