level 3 heart murmur from nothing in well cat in 6 weeks?

I took my neutered male 12 yo cat to my vet for a cut paw. She announced he had a level 3 heart murmur. But no issue was found in his annual checkup only 6 weeks before nor in the previous 12 years of checkups at the practice. Can a healthy mature cat suddenly develop a serious heart murmur out of nothing in only 6 weeks?  He is lively and well and has never had any illness. I would be immensely grateful for any guidance - I will be taking him for his next health check shortly and am considering a different vet for a fresh pair of eyes as I no longer trust my current one.  (For context, this lady stated with pride how she could spot heart murmurs where other vets can't, and I've also read how cats can mimic heart murmurs due to stress.).

Thank you for this excellent and interesting question, which illustrates a point that even good vets often overlook; that owners do privately question what we say without voicing it in consultations. The questionning, for a client, is like having a stone in the shoe; it affects how comfortably you walk with your vet going forward.  I have experiened the same with Drs.  It's a great that you've reached out;  communication issues are the main reason for clients to seek other vets. If there is a vet at your practise who does have more sensitive than usual hearing, this is a bonus rather than a sign that your usual vet is inadequate, although I would also encourage vets to be aware that deteriorating hearing can be an issue.  You could ask your first vet to check their ability to hear murmurs on a known heart-patient just in case; in my experience, vets as a "type" are very sensitive to criticism and would probably do so.  However, I would expect this to relate more to the quiet, barely-perceptible gr 1 murmurs and wonder whether the second vet, faced with perceived criticism of their colleague unexpectedly, simply hadnt thought it through very hard on the day because I agree that at grade three, this is less likely.  I would however suggest an alterntive explanation; that murmurs are simply not audible all of the time.  Patient stress, for example, can affect blood pressure and high blood pressure can easily alter the presence of a murmur.   Even a glimpse of a dog in the waiting room might make a patient more anxious.  This is one way that cats can 'get' heart murmurs due to stress; the murmur was always there, but the stress exacerbated it.