There is no condition called "sitting and being massaged", not even one for sitting (no massage). Your DM will have to adjudicate that.
For what it's worth, flat-footed seems to be about the best fit from the available list of conditions, if you want to use one of those. The penalty effect is: "You take a –2 circumstance penalty to AC." Depending on how positioned, heavy and serious about their work the slave is, maybe even grabbed -- but from your description (sitting, not lying), it sounds like that would be overdoing it.
If they were lying on the sofa, maybe even on their stomach, the DM might use prone condition instead. While it says "lying on the ground", so technically they are not prone, they are in a situation that would be quite similar in regards to the penalities they should experience trying to get up or defending themselves when attacked. But if thay are just sitting, getting a neck massage or something, they are probably able to easily jump to standing, so this would be too much.