The key to this cover-up is that the next E-Class is expected to share the same modular platform with the C-Class. That makes it relatively easy for Mercedes' engineers to mock up the larger sedan and send it out for testing with a modest amount of camo. In this case, the wider fenders are easy to spot, but it takes an eagle eye to also spot the added length. If you look at the test car in profile, notice that there is extra room in the length of the hood and in the area between the wheel well and front door, compared to the standard C-Class.
Earlier rumors put the launch of the new E-Class in 2016, possibly to coincide with the debut of Mercedes' rumored, new inline-six engine. If true, that might make this a fairly early prototype for the next E. We expect the sedan, coupe, convertible and wagon variants to all continue to be offered, but of course, Mercedes has yet to confirm any of them as of right now.