Currently the "Show route" button for an aircraft with an undefined source/destination is deactivated (greyed out).

I propose that "Show route" should simple show the currently known route, which is the route from the first radar appearance until now, without the future projection, which is of course not possible if the system doesn't know the destination.

With these function one could simply guess where from the aircraft originated and what route it took so far. Thanks!

The current workaround is to zoom out manually (up to regional/continental scale), with the negative side effect of having many objects on the map (slowing down the browser tremendously). The more performant workaround would be to first enable the filter for your current plane, then zoom out. But then this is not a quick function, but a whole complicated procedure. If you suspect i.e. that multiple close aircraft with a similar heading are belonging together, using the "Show route" function on them successively would quickly answer you that question if they have a similar route and/or origin.