Not hidden perfectly, or I wouldn't have spotted it:
Olympus Stylus 1s, 300mm, ISO200, f5.6, 1/250s
I always find it very hard to determine from which species eggs are, so I opted for the easiest way to find out, which is to wait for mum to arrive:
Olympus Stylus 1s, 300mm, ISO200, f5.6, 1/250s
She turned out to be a waterhen (Gallinula chloropus), waterhoen in Dutch.
These waterhens have the same build as coots, but the red-and-yellow beaks make it easy to tell them apart. For some photos of a coot, look in this post featuring Lakeisha Coot as herself.
Thanks for watching!
PS It suddenly dawned on me why these birds wait until well after Easter before laying eggs in hidden places.