K.M.
You are absolutely right to expect recognition when you drop off your son. I don't have my daughter in a daycare setting, she's in a sitter's home, but I would always want someone to at least greet my child when I drop them off. That doesn't provide much of a sense of security for either of you.
Playing devil's advocate- Is there a different teacher? Are there suddenly more children in the room? Have there been other issues with other children the teachers might have been tending to?
I would bring it up to the daycare director and see if it can easily be rectified. Good luck!