I would think they would know which vehicle from the Walmart security cameras.
I think they would know for sure what kind of vehicle they were in from the McDonald's drive-thru camera. It gives the person inside taking the order a clear view of the car and the occupants. Usually there is an indication on the ticket of the type of vehicle (car, truck, minivan) and the color (which might be a little off depending upon the lighting - like a dark blue car could be tagged as black, for instance). Obviously they would also know if MR and DR were alone, or with anyone else (in regard to the "we").
----------
One thing I wanted to mention in regard to the "we" saying... I noticed that I tended to do this recently (w/in the last couple years), and I came to the conclusion that it made me perhaps sound a bit daft (unless people I was speaking with knew my husband and I, and I was speaking on behalf of both of us at the time).
I also realized when I said "We'll see you later..." it wasn't referring to an extra person it was just a tremendously slangy version of "Well, I'll see you later" (not sure when I picked up that habit - I grew up in the NW suburbs of Chicago, and I lived in the Denver area for almost 10 years so I don't know if it's a "me" thing, or a regional thing for either place...).
So, I for one, can see MR using the word "we" in place of "I" even if there was no-one else with him. Not all of the reasons a person uses "we" are indicative of having an extra person along, however I do seem to use it myself when I want to include my husband when passing along hello's, or goodbye's and such. I remember MR referring to ER as "Mrs. R------" at the DP show though, and it seemed to me to be a possessive type statement, so perhaps he is still thinking of she and he being together when it comes to DR???
As always, all of the above is MOO! :cow:
ETA: I think part of my using "we" is that I have 4 children who used to be with me
all of the time for a great many years starting 21 years ago! Only recently have all of them been old enough where I have been out by myself, and even now I usually bring a child or two along with me to places I go...