I am very confused!
Cat has a litter tray, which he uses regularly, and also had access to outside (poos out there occasionally, but usually inside). He seems healthy, and happy (although he does sometimes get bullied by other cats that come into the garden). This morning I found two pieces of (thankfully dry!) cat poo on different steps of the stairs
. This has happened before, a month or so ago (we cleaned up with "wash off" spray). The stairs are wooden and have open banisters so I can't see why they are attractive 
Any ideas why he has done this??