Emergency tyre sealants are designed as a last-resort temporary fix in order to make the car mobile again, and are not designed for long-term use. The fact that no leak was found is likely just that the sealant was effective, but it does not make the tyre entirely safe to use. Vehicle tyres are designed to be inflated with air, and how they behave on the road is dependent on a consistent volume of air within the tyre. The sealant has stopped the air escaping but this does not mean the tyre is structurally safe, and while the difference may be small the fact that part of tyre is now filled with something other than air means the tyre is unbalanced, which can be problematic in itself. Once a tyre has been filled with sealant it should be replaced as soon as possible and the original tyre disposed of.