“maybe someone had walked their dog there and allowed / encouraged it to urinate on the monument”
That was my first impression also. The height of the “markings” offer no other conclusion. And, yes, dog urine feels oily.
Urine oil is possible, but in small quantities.
It was found out today that the “substance” was also on the two Memorial Statues at the WALL. That’s a good bladder if you are correct.
tgslTakoma has taken a video of all three sites. Hopefully it will be up tonight.
[Mr] T