Does Brussels Airport have a lost and found counter? If yes, where can I find it and what's the schedule?
I looked it up on their website and only a form is provided.
As the OP has discovered The Brussels Airport website has a very comprehensive page dealing with lost and found.
In answer to the question about a lost and found counter, the answer is not specifically. In the airport, lost and found queries are dealt with by the airport information desk, which is located in the arrivals hall, and as of the date of answer weekday opening hours are 06.00 to 22.00, closing one hour earlier on weekends.
From the OP's comment about a missing item in his luggage, a baggage report with their airline's handling agent should also be filed.