in my book, there is a 20 page appendix that lists out all the evidence that the French had trickling in about the Prussians. (this appendix is written by Coppens favorite person, ha! But the evidence is as sound as any from 1815.)
There I checked Coppens book precisely about that topic I would be surprised about an appendix of 20 pages, additional to Coppens Oliver Schmidt pointed out other vital Prussian sources (like of advance Guard leaders) whose reports confirm the total suprise for the French when they entered the fight.
They would have delayed their attack against the French even longer - but from their point of view - the army of Wellington was in dire straights and they (Blücher and Bülow) came to the conclusion they couldn't wait any longer to support Wellington, despite only two brigades were concentrated.
To sit - observe and wait leisurely - till the time chosen to engage (not due of French actions against them but indirectly French success against Wellington) - is an indication for me (as well as the Prussian reports) that Napoleon didn't undertake any stept to interfer much earlier against the Prussians and to use the terrain most favourable to him to do so (the defilee at Lasnes).
There was a good discussion on napoleon-online.de about this topic where all arguments that Napoleon knew already at 12:00 that the Prussians were there - crumbled.