Definately old poo and wee, and i agree with putting it in the same spot they chose.
I did this then after about a week i moved the tray to somewhere better suited, and the buns just learnt that the tray was the place to do it, rather than the origional spot. I put a bowl wiht some veg in it when i moved it, they LOVE their veg so this really helped them, and not one accident since!
It may take a while or it could be very quick, one of my buns will wee fine and do maybe half of his poos in the tray, but the other half he just hops along and poops, whilst one of my females did it from day one and not even one poo out of the tray when shes in the cage or house, but poos in the garden, which is fine by me!
Good luck!