From what I am reading, this mechanic is not competent to do electrical diagnostics. It is an art and not all mechanics are able to do it.I believe last time the mechanic said this might be the issue already unplugged it. Right now it doesn’t work anymore.
If you have a functioning kessy and unplug it, the vehicle will not start.
Only the kessy with the touch door handles goes bad and drains the battery. If you don't have the touch door handles, you need to be looking elsewhere for your drain issue.
If you have bad kessy, you will get antenna errors on the vcds scan. When the battery drain gets bad, it will kill a battery overnight.
Fixing a kessy is what, about $25 worth of parts. The tough part is finding someone to solder the new parts on the old kessy's circuit board.
The way you find out if your kessy is bad is by scanning it for errors. If you have no antenna errors, then it is something else draining the battery.
A good shop could easily find the battery drain culprit in a couple of days.
A VCDS scan can tell you if it is the kessy antenna issue, as the kessy will post an error.