It really makes you wonder - why didn't the allies make some of those "significant troop movements," maybe catch the French in the act of crossing the Sambre, and wipe them out piecemeal?
Because they were surprised. Surprised not in a startled type of way, but surprised in the military sense that while the enemy was concentrated and maneuvering, theirs was not.
They were not suprised but also they couldn't mind read. An offensive that early in this campaign would be to act piecemeal as well, open up any cohesion and brake up all lines of communication of the Allied Army in Belgium (the more I think about it - the more I am confirmed that Wellingtons and Blüchers army were left and right wing of an army, acting a bit like the Trachenberg plan in 1813).
The Allies very well knew how to beat Napoleon from their expierences in 1812 and 1813 - they achieved an edge in military stretegical planning which they kept.
They acted very promptly, despite all the what ifs - this caused frustration for Napoleon at Quatre Bars and by hindsight also Ligny (where he thought he beat the Prussian Army thoroughly - but which was in the end - a hollow victory).