Got it! The footprints are quite subtle, and I wouldn't have noticed them if I weren't looking for them. I realize that is by design, and I like it, but I think the goal should be noted in the game description and on the in-game instruction screen.
Superb presentation. Excellent graphics and sound. The music is upbeat but doesn't outwear its welcome, and I could listen to the bubbling noises all day. The lighting is also subtle and well done.
The upgrade system between rounds is interesting. I am not sure whether my assistant was adding any value--I didn't have time to supervise him! :) More variety in challenges (e.g., litter or small fires) would also be fun.