Very true about "Track Circuits" and we owners of speeders know that because we would have the same problem if it weren't for the fact that our speeders are insulated so we won't intermittently operate gates.

In this case of a train disappearing and being hit, then during the test when parking a similar train in the same spot and having it disappear, is highly likely that something else is going on. To have a dirty spot in the same place two times back to back indicates some other problem.