I'm inclined to agree with Gill, Helen, although I can understand your concern as I have a similar situation with both Jaffa and Mosi.
Although there was nothing found, the fact that your vet ordered tests could be used by an insurer in the future to claim that he was exhibiting symptoms prior to taking out a policy with them. If he developed a heart murmur in the future, for example, (Magpie was diagnosed with a mild heart murmur age 6 of 7), then an insurer would ask when symptoms were first noticed and/or investigated. Even though his panting was not found to be due to anything abnormal, they might consider the panting and subsequent investigations as relevant to the later condition. I wouldn't agree with that decision, but it could make claiming difficult. I suppose the best thing would be to discuss it on the phone with a new insurer and explain that it was only investigated in case it was a symptom of something, and that all the tests were clear. They may be fine about it and not class it as a pre-existing condition.