What plant have I collected?
You definitely have a seedling with prominent cotyledons still present.

My initial guess is Acer pseudoplatanus (sycamore maple, or just called "sycamore" or "planetree maple" in parts of Europe)



 Source: Wikimedia 

You can see a detailed image of an A. pseudoplatanus seedling here and a time-lapse of a seedling growing to the point of cotyledon release in Neil Bromhall's youTube video.

According to the range map available from Euforgen, A. pseudoplatanus is present (blue) throughout Belgium:



As for confirming nearby adults, you're looking for a tree that looks as follows:



 Source: Blue River Nursery 

For more information regarding identifying the adult, please see Virginia Tech Dendrology