Okay this exact situation happened to me. Vendor in an underdarl market (shady type market that sells slaves and poisons) has a medium-sized lock box underneath a table. PC casts invisibility, and stands in front of the vendor (there's no room to get behind her since the stalls are up against a wall and each vendor is tightly packed in).
PC then summons a dolphin in the middle of the market to cause a distraction (points for originality here). The guards surround the dolphin confused, but figure out a caster's in the area. All the vendors instinctively put their hands over their merchandise and lockboxes.
The PC invisible has a +20 to the sleight of hand roll -- his total is 39.
There's no rules for a medium sized object (not to mention he's leaning over the stall to reach under it and pull the lockbox from underneath the vendor's hand).
Vendor's perception roll was 18. I figured even with a -20 that meant he got it out from underneath her hand. But then he has to bring a medium-sized lockbox floating mid air past the nose of a vendor -- aware that something suspicious is going on -- before he can tuck it in the folds of his cloak? Yeah, she noticed that.
PC got a bit upset - cos how could she notice on such a great roll. Given the circumstances, I think it was the right call. Opinions?