Cat - Murmur

I recently took in an outdoor cat (previously a neighbor's cat who was moving and couldn't take him in) and transitioned him indoors. I took him last weekend for a nail trim, and the vet did an exam and said he had a grade 3 murmur. 6 months ago, he was at an emergency vet and a regular vet for a bite wound by another cat prior to being indoors and neither notated a murmur. Can there ever be false diagnosises for a murmur? I'm barely making ends meet, and an echo is so expensive 😭 I'm raising funds and almost where I need to he to get him checked, but find it so odd nothing was noticed previously 😞