I was thinking something along these lines. If the pt's cognition is not enough to use a PCA, then that would be documented in your assessment. It would just take 'putting 2 and 2 together' to figure out that the pt wasnt hitting the button.
The only time I hit the button for the pt is if it is totally out of the pts reach, ie. he's getting his bed changed, in transport/transfer in CT, that sort of thing. And in any instance like that I would ask the pt, 'do you want me to hit your button for you?'
....and of course, I try to premed before procedures, dsg changes, ect and have them hit the button before I move it away from them, i like to clip it to their id bracelet so it doesn't get away, always educate the family on proper use of the pca; and for my #1 pet peeve.... get them on PO meds asap!