You are Welcome.
This morning I was walking into the main building, here at work, when i noticed someone was on its way out. Like you do, I held the door open, the person said, “Thank you Sir” to which I replied “mhmm!” (usually with a smile too) and then it hit me. That is usually my auto-response to everyone that says “thank you” to me, not always but most of the time. It comes out without me even thinking about it at all. Now that I am thinking about it, it might come out a bit disingenuine, I think, or just plain laziness when I should reply “You are Welcome” not just “mhmm”.
It is the same way as when someone says “Thanks” as supposed to “Thank you”. To me, it feels lazy and a little bit disingenuine, just like “Mhmm”
Gotta work on that.