Is there mud anywhere else in the house, or just Sergio's room? If just in the room, then it is very possible that it is being staged as there would be mud outside his door. Check to see if any shoes in the house may have left the footprints, including what the mother wears/is wearing. If this is the case, either Elena is the prime suspect in the case(A), Sergio ran away(B), or the kidnapper(s) planted the evidence.
(A)Confront her and, assuming she caves, arrest her. Interrogate her to find out where he is/what she did with him. If she appears innocent, continue on.
(B)He may even have had friends stage it, so ask Elena about his friends. Check his drawers and personal effects as well.
(NOTE) Assuming it is a kidnapping, it is imperitive to lay low as police/detective involvement may cause Sergio's death (assuming he's still alive).
If there is mud elsewhere or Elena cleaned it up without realizing Sergio was gone, check to see if any shoes in the house may have left the footprints, including what the mother wears/is wearing. Ask if Sergio may have had shoes that match the prints. If there are no matches, take a mold of the footprint and determine what type of shoe it is, and check for anyone who has recently bought some. Also check around the house for anything unusual (a trained eye might find something others miss.