Strange scab on neck?

Hi I was petting my cat at the back of her neck and felt this (in picture) thought initially it was something stuck in her fur then I realised it was some sort of scab, it looks like dried blood? I took this picture and we are going to book a trip to the vet to see what this is but does anyone here have ideas? She’s an indoor cat, there’s no sign of fleas or anything else that we can see. I’m unsure if she’s somehow scratched herself and this is just a cut healing but given it’s position I don’t think she could’ve scratched herself here. I’m concerned since the last time she had something similar but smaller it was after she received a vaccine and became really sick leading the vet to think she had an allergic reaction (this was about 3 years ago). I’m worried it’s perhaps some sort of allergy again but we haven’t changed anything diet wise etc. so I’m really not sure and concerned. As I said we have a vet appointment booked, but if anyone has any ideas on what this could possibly be that would be helpful. Thanks.

Hello!  That looks like dried blood to me from here too, but your vet will get their fingers in there and check for lumps / cysts / tags / ticks etc. vs scab.  You can't rule fleas out on the basis of a cat being an indoor cat because fleas have seemingly impossible skill in penetrating households.  I have seen cats manage to scratch right across their necks when necessary, but a good quality flea treatment will cover it (you might want to wait for your vet with that).  Fly bites are another possibility - presumably cat-bites aren't possible?  I would love to hear what your vet thinks when they can examine it!