The indicators stay on on my Vauxhall Vectra?

I have a Vauxhall Vectra on a P reg. It started off when I locked the car and left it, sometimes I came back to find both indicators were on (not flashing). It's not happening all the time. But if I had left it more than an hour it would drain the battery, and I would be left stranded.

It is not, as answered by another person with the same problem, that I didn't cancel the indicators before I left the car.

I didn't leave the indicators on.

Now I don't lock my car at all but the indicator lights still come on. Sometimes I will have been home for maybe 4 hours and then they come on (I swear there is a gremlin in there).

I have had an electrical guy out twice. He has replaced the fuse board/box and disarmed the alarm, neither of these have worked.

Please do you have any ideas. I don't want to get rid of my car because other than this it has been a good car and is a good runner.