Liz Buchanan BVSc MRCVS
Hello - Given that this is apparently painless, I'm afraid that various sorts of cancer may be quite high on the vets' differentials (possible diagnoses) list. They may also list some infectious agents such as Ringworm, traumatic wounds eg bites from rodents and so on. Cats can be quite bad at showing pain, even when they feel it, so pain releif might be encouraged anyway. It is definitely worth going to see then very soon, to get your cats' nose assessed and make a plan together, specific to what the vet can see when they look at this in context. Wishing you and she all the best.