I've read several threads about horse theft - all dreadful! I just thought 'd pass on the following link I found on line:

The Courier - Horse owners warned mystery plaiting may signal thieves

The plaits are usually very small. Although it relates to Scotland, this sign has been seen all over the country.

Horsewatch run a missing/stolen horse register:

Stolen Horse Register

Always worth checking the register if you are thinking of buying a horse you don't know.

Horsewatch : UK Horsewatch Alliance

i had my horse stole then found her ! was told to go to court to get it bk what has this world come to!!!!!

Some of it is caused by 'the wind' and some people doing it as a wind up.

Before too long, the plaits will stop now it is getting more known about what they are often caused by. Saying that, if people do see 'tagged' manes, still report it and be vigil!