Somethimes the areas around the entrances are a little bit brighter than the rest, this due to the bellows of the gangways, they cover this part and the residual aircraft gets dirty by the soot.
Other possibility's would be (as already posted) some maintenance work / inspections ect.
greetings from Munich
PS: my English-skills aren't the best an i try do get better, so if you think i did "major" mistakes in my explanation -> let me know
