Occasionally horses that are leased "off-farm" or "off-site" do disappear. One way to avoid that happening is to be in constant contact with the person leasing the horse AND fully vet the leaser before they take the horse.
Contracts are great, and are pretty necessary, but they are not a guarantee that bad things aren't going to happen. I would not lease on an open-ended contract. I would require it to be reviewed and re-signed every 3-6 months. That gives you an opportunity to check in on the horse and it's condition.
"On-site" or "On-farm" leases are generally much safer. Especially if the horse is kept on your property, or at your boarding barn where it can be checked on often. This type of lease also allows you to monitor how the leaser interacts with the horse.