You see it often, dealers stating that they have had the watches they sell 'serviced'. But serviced by who? To what standard? Dealers often work to tight margins, a genuine full service would push prices up further. The biggest, Watchfinder has a wonderfully obscurantist paragraph which avoids actually saying their watches have been serviced. So they probably haven't.
Others do make these claims without real details. I bought a lovely IWC which, I was told, had been fully serviced. It turned out that the watch had no lubrication and hadn't been serviced for many years. This from a major dealer in used watches.
Who can you trust? How can you tell? Personally, if the watch doesn't have a service receipt from a good technician, I ignore all claims of 'serviced.' Too cynical?