People regularly flashing their lights at me?!

[BeBi]

Hi, I have a Yaris. For months and months we have been getting people flashing their lights at us when they are travelling behind us. This has some times gone on for over 5 minutes until we can get to somewhere safe to stop and check. Each time we stop and look though, we can't see anything wrong?! It has happened both day and night, so I am struggling to see what is going wrong. We have just been outside again and checked, and every light seems to be coming on ok. The car has also had at least 2 services and an mot since we first noticed it happening, and the garage hasn't picked anything up either. Has anyone else experienced anything like this too???